Re: problems with compiz and multiple cards
not sure if this covers what your asking but i run 2 x 8600GTS with 4 screens, each card has twinview enabled then xinerama binds the 2 screens together into the full desktop, i have no performance issues with it and its common for me to have 2 to 3 openGL apps running on different screens at one time, there is the smearing when dragging a window accross the GPU boundary but that is purely cosmetic and disappears when you let go of the window. Yes you can leave a video or 3d app scaled across the GPU boundary and it will continue to run, doing this with an eve-online client drops my frame rate by about 10-15 FPS (50FPS instead of 65 or something) which comes back as soon as the client is entirely on 1 window
xinerama does not support compositing and most likely never will, you could try to get xgl to work but that is a dead project and likely has incompatibility problems at this point.
The proper solution will come with "GPU Object" support from the XRandR guys, it was planned for randr 1.3 but may have slipped as intel pushed the new X server version of the door.
Until then, full 3d performance = yes, compositing = no