Most games run better on Windows than on Mac. Its just because Apple doesn't allow the same amount of customization for Gamers and Game Developers, especially with drivers.
As an example: Civilization V runs way better on Windows on my machine (Ultimate 13'' Air). On OS X I need to turn off a lot of details to play it (it works fine besides that).
On the other side, Starcraft II runs smoothly on 1440x900 and Medium/High details. Awesome job done by Blizzard.
I find almost all Steam games play better on Windows in Boot Camp than native Mac OSX versions on my MBA13. Same goes for a few MMO games. In fact, one MMO in particular CRASHES consistently under OSX native client, while the Windows version is rock solid.
Generally speaking, I find gaming under OSX to be pretty craptacular.
Yes Starcraft 2 seems to be an exception. I ran it both on Windows 7 and OSX 10.6.6 on my iMac i5, and the frame rates were pretty equal (+/- 5FPS) with the same details/resolution/graphic options.