Thanks Blair for the information.
Well if NVIDIA is really faulty, I can tell you their drivers are really far off from their Windows ones.
I compared Bioshock Infinite on Windows on my iMac, and it's DAY AND NIGHT, really, with the naked eye you can see the performances are different.
Let me explain :
First of all, on Windows I can set it to Ultra and native resolution without tweaking any any, thus it looks much nicer. That being said, everything maxed, the game performs better than the OSX version in the maximum resolution (900p ?) in High. While the OSX version crawls sometimes to 30fps, the Windows version manage to achieve in Ultra + high resolution an overall of 50fps I'd say.
If I set the Shadows to High instead of Ultra, then the game performs at 60fps in almost EVERY scene, which is quite amazing. I didn't expect it to run so nice on this iMac, but it does.
Now you can see why I'm sad about the OS X version. As expected, the OS X version performs a lot worse than its windows counterpart, and if the NVIDIA drivers are the only reasons of this difference, then you must slap NVIDIA
