Apple has implemented updated graphics drivers for the newer iMac models, and released a graphics update for all other of their most recent computers, so graphics performance should go up majorly. Regardless, the graphics performance still isn't as good on the osx side than the windows side. If you wanna max starcraft2 out, I recommend gaming under windows. When overclocking the mobility 4850 in my iMac by 20%-- to bring it to desktop 4850 standards, I can max out starcraft2 at native resolution on windows (averaging around 50fps for 1-4player battles and 30fps for in-between mission scenes on campaign). Your iMac has a mobility 5850 card, which is around the same performance as a desktop 4850, so you should be able to max out sc2 at native resolution on windows side (you can also overclock it to bring it up to mobility 5870 standards extremely easily)
Also, another thing you can do that's extremely efficient is to have everything maxed and native resolution on starcraft2, spare shaders to high instead of ultra. Everything outside of between mission scenes will look the exact same, and you'll see up to a 10fps performance increase.