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Unity3D : Performance with multi-camera rendering with a lot of spheric meshes

I have an Unity 3D scene with several cameras looking at the same object (a huge brain mesh ~100k tri) but not necessary with the same point of view.
In the same 3D scene there is a huge number of spheric plots meshes (from 100 to 30000).
In all the cameras i have to display the brain mesh with a part of the plots meshes.
Depending on the camera view, each plot can have a different size (mesh filter and spheric collider), a different material (opaque or transparent) and can be visible or not.
The spheric collider must have the same size than the mesh.
I set up a shared mesh in common for each spheric mesh.
Their material can be one of the several shared materials i have defined.
Before rendering the scene, for each camera view in the OnPreCull function i have to define which plots are visibles and how they look.
This part can be very costly, i tried several things :
setting gameobject inactive : too costly
setting local scale to vector3(0,0,0) : better but i can see that the rendering is still done in the profiler
setting a total transparent material : same result, but the in the profiler the rendering is now transparent instead of opaque
setting a layer not in the cameras layers masks : huge script cost
I don't kwnow if i can make an efficient culling system with all theses cameras looking at the same point...
I welcome any new ideas.

First issue:
Regarding your specific question with the four dots.
Simply set the renderer.enabled = false, that's all there is to it.
Note however that as I mention in a comment, you would never try to "cull yourself" in Unity (unless I have misunderstood your description).
Second issue:
Regarding the small spheres. I suspect you have very many in the scene. You simply can't do that. In video games (the most difficult of all 3D engineering), you just do this with billboarding. It's how say "grass" is done in a scene. You can achieve this nicely with the particle system in Unity, or other techniques. An implementation is beyond the scope of this answer, but you will have to fully investigate billboarding. Simply it's a small flat image which always faces the camera in the render pass.
Issue 2B:
Note however that sphere colliders are wonderful, and you can use as many as you want. I'm sure this is obvious from base mathematical reasons. Side tip: often folks try to "write their own" thinking it will be faster. It's impossible to outwrite the 100? person-years of spatial culling scientific research in PhysX, and moreover they use the metal, the gpu, so you can't beat it.
Issue three:
Is there a chance you're using a mesh collider somewhere in the project? Never use mesh colliders, at all. (It's extremely confusing they are mentioned or used in Unity; they only have one or two very specific limited uses.)
Issue four:
I'm confused about why you are turning things on and off. I have a guess.
I suspect you are not using more than one "stage"!
There's an amazing trick about video games when you have more than one camera. In fact you have "offscreen" scenes! So you may have players in a dungeon or the like. Off "to the side" you may have an entirely duplicate or triplicate setup of the whole thing running (you could "see it if the camera turned the wrong way") for the other cameras. (In the example you would have different qualities on the dopplegangers, coloring, map-style or whatever the case is.) Sometimes you make a whole double just to run physics calculations or address other problems.
Fascinating extreme example of that sort of thing.
In short in your situation,
You likely need one whole 'stage' of a camera and brain for each of the camera views!
Again this can be http://answers.unity3d.com/answers/299823/view.html but it is indeed the everyday thing. In your overall scene you will see eight happy brains sitting in a row, each with their own camera. In each one you would display whatever items/angle etc are relevant. (Obviously, if certain items are "identical, other than the viewing angle" you could use the "same brain with more than one camera": but I would not do that, best to have one-brain-one-camera for each view.)
I believe that could be the fundamental issue you're having!

Getting "giggly" effect when slowly moving a sprite

How do I remove this "giggly" effect when slowly moving a sprite?
I have tried adjusting Antialiasing values in QualitySettings and Filter Mode in ImportSettings in the Unity Editor but that doesn't change anything.
Ideally, I would like to keep the Filter Mode to Point (no filter) and anti aliasing turned on to 2x
The sprite is located inside a Sprite Renderer component of a GameObject.
I have uploaded my Unity Project here: http://www.filedropper.com/sprite
I really don't know how to fix the problem... Can anyone help with my personal project?

I cooked up a quick animation to demonstrate what's happening here:
The grid represents the output pixels of your display. I've overlaid on top of it the sliding sprite we want to sample, if we could render it with unlimited sub-pixel resolution.
The dots in the center of each grid cell represent their sampling point. Because we're using Nearest-Nieghbour/Point filtering, that's the only point in the texture they pay attention to. When the edge of a new colour crosses that sampling point, the whole pixel changes colour at once.
The trouble arises when the source texel grid doesn't line up with our output pixels. In the example above, the sprite is 16x16 texels, but I've scaled it to occupy 17x17 pixels on the display. That means, somewhere in every frame, some texels must get repeated. Where this happens changes as we move the sprite around.
Because each texel is rendered slightly larger than a pixel, there's a moment where it completely bridges the sampling points of two adjacent pixels. Both sampling points land within the same enlarged texel, so both pixels see that texel as the nearest one to sample from, and the texel gets output to the screen in two places.
In this case, since there's only a 1/16th scale difference, each texel is only in this weird situation for a frame or two, then it shifts to its neighbour, creating a ripple of doubled pixels that appears to slide across the image.
(One could view this as a type of moiré pattern resulting from the interaction of the texel grid and the sampling grid when they're dissimilar)
The fix is to ensure that you scale your pixel art so each texel is displayed at the size of an integer multiple of pixels.
Either 1:1
Or 2:1, 3:1...
Using a higher multiple lets the sprite move in increments shorter than its own texel size, without localized stretching that impacts the intended appearance of the art.
So: pay close attention to the resolution of your output and the scaling applied to your assets, to ensure you keep an integer multiple relationship between them. The blog post that CAD97 links has practical steps you can take to achieve this.
Edit: To demonstrate this in the Unity project you've uploaded, I modified the camera settings to match your pixels to units setting, and laid out the following test. The Mario at the top has a slightly non-integer texel-to-pixel ratio (1.01:1), while the Mario at the bottom has 1:1. You can see only the top Mario exhibits rippling artifacts:

You might be interested in this blog post about making "pixel-perfect" 2D games in Unity.
Some relevant excerpts:
If you start your pixel game with all the default settings in Unity, it will look terrible!
The secret to making your pixelated game look nice is to ensure that your sprite is rendered on a nice pixel boundary. In other words, ensure that each pixel of your sprite is rendered on one screen pixel.
These other settings are essential to make things as crisp as possible.
On the sprite:
Ensure your sprites are using lossless compression e.g. True Color
Turn off mipmapping
Use Point sampling
In Render Quality Settings:
Turn off anisotropic filtering
Turn off anti aliasing
Turn on pixel snapping in the sprite shader by creating a custom material that uses the Sprite/Default shader and attaching it to the SpriteRenderer.
Also, I'd just like to point out that Unless you are applying Physics, Never Use FixedUpdate. Also, if your sprite has a Collider and is moving, it should have a Kinematic RigidBody attached even if you're never going to use physics, to tell the engine that the Collider is going to move.

Same problem here. I noticed that the camera settings and scale are also rather important to fix the rippling problem.

Here is What Worked for me:
Go to Project Settings > Quality
Under Quality Make the default Quality as High for all.
Set the Anistropic Texture to "Disabled"
Done, And the issue is resolved for me.
Image Reference:
enter image description here

WinForms Draw parts of image at a rotated rectangle frame

I'm working on image transitions for my digital photo frame and am trying to achieve this transition:
It's more of a radar-style transition with the wiping effect going from one side to another in a 180 degree angle. Although, it doesn't appear that "blocky", I just spaced out the rectangles for illustration purposes. The entire thing should be a smooth transitions without any FPS stuttering effects.
My logic is to draw the specific part of the image at (theta) rotation angle like my drawing above - but that will end up drawing 100's of rectangles that sweeps across the screen.
Is there a more efficient way to do this? If not, could I have a few code tips to point me in the right direction?

It is practically impossible to have without any FPS shuttering especially in bigger screens because WinForms uses CPU only rendering. You will have to embed OpenTK (if you want to use OpenGL) or Direct3D frame inside, or maybe WPF where you do the animation.
If you use any of them (for example OpenGL), you have to load it as a texture, and the animation would be done on the triangle level (dragging the corners only) not on the image itself.
If you want to have a curved surface, like a real page transition, I recommend to use a bezier patch as is found here: http://nehe.gamedev.net/tutorial/bezier_patches__fullscreen_fix/18003/
This coding takes a lot of time, and is much more over the purpose of StackOverflow (to setup a full OpenGL/DirectX control + how to do a Bezier patch if you want to set it up).
If you don't want to embed anything, you may look to this transformations tutorial using WPF, but I'm not 100% sure that this is what you need:
http://www.codeproject.com/Articles/14895/WPF-Tutorial-Part-Transformations

How to render specific edges of a cube different from filling in XNA? (Monogame)

I am working with C#, Monogame and XNA 4.0. In my scene I have a lot of cubes. Some are connected, some are not. I would like to render the edges of the cube with another shader than the filling. Besides that, I would like to render the outer edges of connected cubes in another color (or thicker) than the edges within the cube-object. Here is a small painting to make clear what I want to do (sorry for my bad painting skills, but I think you will get it).
I know how to render a cube with a specific shader and I am also able to render the wireframe but I was not able to connect both methods. Besids that, the outer lines can not be rendered differently with this approach.
I tried it with post-effects like the edgefinding of comic shaders but in this approach I am not able render only specific edges. Besides that if two cubes are next to each other the shader does not recognize the edges.
I am not searching for a ready-to-use solution from you but I would be glad to get some tips/approaches/tutorials/similar projects/etc on how to achieve my goal. Are there some shader experts out there? I am at my wit's end.
(If you however would like to post a ready to use solution I would not be miffy :D)

It is a shame you're not using deferred shading, this would be pretty straight forward to implement if you were.
If you can access the normal and material for each pixel on screen through a texture lookup you can easily post-process this. You could use a 3x3 filter kernel and search for sufficiently large normal discontinuities (this would catch silhouette edges) and also search for pixels that lie on the transition between material IDs (this would catch the edges between blue and orange cubes). If your filter neighborhood satisfied either of these two conditions, then draw a black pixel to form the outline.
You should be able to do this if you use MRT rendering when you draw your cubes, and encode the normal + material ID into an RGBA texture (x,y,z,material).
The basic theory is described in this paper (pp. 13). In this case instead of using the depth as the secondary characteristic for outlining, you would use the material (or object ID, if you want EVERY cube to have an outline).

Drawing massive amounts of textured cubes in XNA 4.0

I am trying to write a custom Minecraft Classic multiplayer client in XNA 4.0, but I am completely stumped when it comes to actually drawing the world in the game. Each block is a cube in 3D space, and it is possible for it to have different textures on each side. I have been reading around the Internet, and found out that for a cube to have a different texture on each side, each face needs its own set of vertices. That makes a total of 24 vertices for each cube, and if you have a world that consists of 64*64*64 cubes (or possibly even more!), that makes a lot of vertices.
In my original code, I split up the texture map I had into separate textures, and applied these before drawing each side of every cube. I was told that this is a very expensive approach, and that I should keep the textures in the same file, and simply use the UV coordinates to map certain subtextures onto the cube. This didn't do much for performance though, since the sheer amount of vertices is simply too much. I was also told to collect the vertices in a VertexBuffer and draw them all at once, but this didn't help much either, and occasionally causes an exception when the number of vertices exceeds the maximum size of the buffer. Any attempt I've tried to make cubes share vertices has also failed, resulting in massive slowdown and glitchy cubes.
I have no idea what to do with this. I am pretty good at programming in general, but any kind of 3D programming or game development completely escapes me.
Here is the method I use to draw the cubes. I have two global lists List<VertexPositionTexture> and List<int>, one for vertices and one for indices. When drawing, I iterate through all of the cubes in the world and do RenderShape on the ones that aren't empty (like Air). The shape class that I have is pasted below. The commented code in the AddVertices method is the attempt to make cubes share vertices. When all of the cubes' vertices have been added to the list, the data is pasted into a VertexBuffer and IndexBuffer, and DrawIndexedPrimitives is called.
To be honest, I am probably doing it completely wrong, but I really have no idea how to do it, and there are no tutorials that actually describe how to draw lots of objects, only extremely simple ones. I had to figure out how to redo the BasicShape to have several textures myself.
The shape:
http://pastebin.com/zNUFPygP

You can get a copy of the code I wrote with a few other devs called TechCraft:
http://techcraft.codeplex.com
Its free and open source. It should show you how to create an engine similar to Minecrafts.

There are a lot of things you can do to speed this up:
What you want to do is bake a region of cubes into a vertex buffer. What I mean by this is to take all of the cubes in a small area, and put them all into one vertex buffer. Only update this buffer when a cube changes.
In a world like minecraft's, LOTS of faces are occluding each other. The biggest thing you can do is to hide faces that are shared between two cubes. Imagine two cubes sitting right next to each other, you don't really need to draw the face in between, since it can never be seen anyway. In our engine, this resulted in 20 times less vertices.
_ _ _ _
|_|_| == |_ _|
As for your textures, it is a good idea, like you said, to use a texture atlas. This greatly reduces your draw calls.
Good luck! And if you feel like cheating, look at Infiniminer. Infiniminer is the game minecraft was based off. It's written in XNA and is open-source!

You need to think about reducing the size of the problem. How can you produce the same image by doing less work?
If your cubes are spaced at regular intervals and are all the same size, you may not need to store the vertices at all - your shader may be able to calculate the vertex positions as it runs. If they are different sizes and not spaced at regular intervals, then you may still be able to use some for onf instancing (where you supply the position and size of a cube to a shader and it works out where to render the vertices to make a cube appear at that location)
If your cubes obscure anything behnd them, then you only need to draw the front-most cubes - anything behind them is just not visible. A natural approach for this would be to use an octree data structure, which divides 3D space into voxels (cubes). Using an octree you could quickly deternine which cubes are visible, and just draw those cubes - so rather than drawing 64x64x64 cubes, you may find you nly have to draw a few hundred per frame. You will also find that as the camera moves, the set of visible cubes will not change much, so you may be able to use this "temporal coherence" to update your data structures to minimise the work that needs to be done to decide which cubes are visible.

I don't think Minecraft draws all the cubes, all the time. Most of them are interior, and you need to draw only those on the surface. So basically, you need an efficient voxel renderer.

I recently wrote an engine to do this in XNA, the technique you want to look into is called hardware instancing and allows you to pass one model into the shader with a stream of world positions to "instance" that model hundreds (even thousands of times) all over your game world.
I built my engine on top of this example, replacing the instanced model with my own.
http://xbox.create.msdn.com/en-US/education/catalog/sample/mesh_instancing
Once you make it into a re-usable class, it and its accompanying shaders become very useful for rendering thousands of pretty much anything you want (bushes, trees, cubes, swarms of birds, etc).
Once you have a base model (could be one face of the block), its mesh will have an associated texture that you can then replace with whatever you want to allow you to dynamically change block texturing for each side and differing types of blocks.