- it's possible to extract independently the R, G and B channels of a texture to treat them separately (with shaders) ?
If you only need to output to screen, then yes, this is easy with pixel shaders. If you want to save such altered textures, you would need the canvas/image manipulator objects.
- if the size of a texture used in a level should be using a power of 2 (128*256 etc.) to increase performances ?
No. General size, count and the display size matter, but a non power of two texture shouldn't affect performance (if the gfx card requires such texture formats, the image you use will be stored as a padded power of two anyway).
- if construct can work with tiling textures the 3d way (you can tile as many time as you wish a bitmap on a defined surface)
Not sure, what exactly you have in mind. The tiled background object does that, but I assume you want to tile some other shaped objects? If so, this is possible indirectly, using the shape as a mask and project other objects like the tb onto it (using pixel shaders).
- if it is possible to post process the entire screen at once (lens vignetting) ?
Yes. Some effects work very well as layer effects, processing the whole background to that layer. Furthermore the canvas object, set to "after drawing" also is suitable as a post processor. (btw: there already is a good vignette shader that ships with Construct)