This OpenGL test at Phoronix on Mac, Linux and Win7 may not be exhaustive, but for Mac and Linux to get beat here is something of an indictment. Hell, Microsoft shows its graphical superiority in OSland here stopping all over Mac. With Steam now on the Mac, there may be more impetus to lift this performance for Maccy people.
Other reviews like this one via CNET also show Windows dominance in the graphics department.
Even so, with Steam on the Mac I wouldnt mind giving things like Torchlight a whirl: