well, i think that at least the opengl performance in windows, linux and mac are going to be nearly the same (perhaps 20% performance difference, not more). There was once an interview with an ati engineer and he said that the new driver will be based on the same codebase.
has someone still this link? i would like to read it again^^. (it was in "the truth about amd/ati" i think, but honestly, i don't know )
Shouldn't be any performance difference if they've got the interfaces to the OSes done right. The Windows and Linux drivers are expected to be based off of the same main codebase, with interfaces to the OS accordingly done. The MacOS version of the OpenGL layer is a slightly different beastie, with Apple providing the userspace layers and the hardware vendors providing mainly the hardware level hooks for the OS and the userspace piece to drive, much more like the MesaGL/DRI code design.