The performance is less than stellar. Using the optimal settings for my machine in OS X, i get about 35 FPS in windowed fullscreen mode without any action going on, like in the very beginning of the game when i am building my first probe. In fullscreen mode, using optimal settings, i get about 50 FPS during the same scenario. The problem comes in when you start doing stuff. I play as protoss and whenever you select a building, say a pylon for instance, you get a massive drop in FPS. Note, nothing else is happening, no battles or anything, this is in the beginning of the game when you just have probes. Just select the pylon and when the pylons area shows up on screen you lose about 10 FPS! At first the drop in fps doesnt seem like that big of a deal, but when you have 4-5 pylons and a try to warp in units to a location, the overlapping warp in areas kills the performance. I have no idea how SC2 will perform in the windows environment using the same system but in OS X, the game doesnt do so well if you play protoss. On the other hand though, the campaign seemed to work ok. Here is a link the graphics settings i used.
http://us.battle.net/sc2/en/forum/topic/224842575