How much mhash/sec are you getting?
Well I'm kinda new to this so maybe my numbers are off. The Diablo miner says I'm getting 80,000-100,000 which I guess is 80-100 mhash/sec. My deepbit mining account says I'm averaging 35-45 though which I think is typically low but about where OSX normally is.
You should be getting way more. I get 110 mhash with a 6750M. Try using poclbm in Windows if you can. Oh and pool sites will never show your actual hashrate. They can't calculate that accurately.
It could just be because you use different aggression levels. OSX basically uses OpenGL for all it's rendering so the -f will affect the hashrate more than in Windows. Something like -f 240 completely destroys my hashrate on OSX but it barely goes down in Windows as long as I am not rendering anything 3D.
I think that it's possible to disable Quartz Extreme (the OpenGL renderer) and to use software rendering. Windows 7 users do something similar by disabling Aero. You should be able to disable or tweak Quartz Extreme with the Apple tool QuartzDebug but I didn't try it recently. Quartz Debug is supposed to be in /Developer/Applications/Performance Tools/ if you have the dev tools. You can probably disable it from the command line too but I am not sure exactly how, everything I find on Google seem outdated.