Like total shit lol. Expected too much, but it can only produce "almost playable" frames at low res/low settings. Someone on another forum said he can get up to 70fps on the same config but I have no idea how the hell you can manage that. I get like 40 or 45fps when just my base + 5 SCVs are on screen, huge dips in battles.
Thats always the problem when people test SC2. They test it in the early levels of single player and low unit 1v1 games.
But, I'd imagine that in windows the 13" MBA should not do too bad.