Before Steam was released for Mac I used Crossover a lot. But the performance was maybe half of the performance in Bootcamp plus loads of graphical bugs. And in some games(HaloPC) the mouse pointer was so bad the game was unplayable. Apparently X11 is single-threaded according to some support article I read. :-/
With Mac-native games from Valve, I get more or less the same performance as in Win7 on the same computer.
Is crossover that much worse on Mac or is it that we're playing different games? I bet the answer is somewhere in the middle. I only use Crossover to play Civ4 on Linux. I reboot to play Skyrim because my video card displays awful banding effects if I'm not in "high" display mode, and crossover only supports Skyrim on medium or low.
With Mac-native games from Valve, I get more or less the same performance as in Win7 on the same computer.