I am a fellow SC2 player!
Uhm, I do use the air for sc2. I have the 13 inch MBA.
Honestly, in 1v1 theres no lagg (200 food vs 200) 2v2 theres some lagg 3v3 its fine, not the best when map is maxed and 4v4, you better finish the game before everyone hits 200 food.
I spoke in terms of low graphic settings.
I dont think the ULV will be good.
320m is barely enough to handle sc2 and constant not lagging gameplay, but seeing that ULV SB Intel looses around 50% in windows and (lets suppose 25%), you are better off with a 320m unless you REALLY need processing power.