I would guess that the overhead in that situation would be dependent on the hit from using Compiz anyway, and then the extra hit from managing geometry for the screens. I suspect the latter is not that much. Compiz probably renders to the surface of some 3D object -- now it would just a couple instead of one. I bet the difference is pretty minor.
I was pretty impressed by Nvidia's driver in my situation. The computer I'm working on has FOUR gpus (later it will be 5 or 6). I can run a giant Quake3 or Wow session across all the screens, and the driver pulls off whatever magic it does and things work fine (great job guys!)
I STILL get good framerates -- it is four 9800 GTs right now.
To benchmark it informally.. Quake3 at 1024x768 was scoring about 550fps. Going up to to a Quake3 session at 5000x2100 straddling all four gpus drops the framerate to 350-370 fps. Not bad!
If only Xinerama + Composite [ + RANDR] worked
I hope one day it does. Seems like some of that business is in flux now with X.org.