Well at least it's a good start.
Now that Steam is available for the Mac there are benchmarks out now comparing performance against Windows.
Bottom line is that Windows is still best OS for gamers as anyone with a clue would already know!
To Valve’s credit, at least on current generation Macs, Steam engine games are absolutely playable. It supports Apple’s whole “it just works” mantra. You’re just better off running them in Windows if you have the option. Although I will admit that the convenience of not having to reboot is sometimes worth the frame rate penalty, at least for shorter gaming sessions. If I’m going to be playing for more than 20 minutes, then a reboot is more than worth it.