Here is your card versus my card: http://www.hwcompare.com/1377/geforce-8800-gt-1gb-vs-radeon-hd-5750-1gb/
(That is your card, yes?)
Your card looks like it beats mine across the board, and yet I'm rarely getting lower than 100fps, and even when that happens almost never less than 60fps. That doesn't make a lot of sense. I'm playing windowed mode, not even fullscreen; though I don't have Aero on at all.
I will say that Starcraft II and AVWW have different needs; AVWW is more about pixel fill rate, etc, whereas Starcraft II is likely to be about other factors including polygon count, shader support, etc, among many others.
That's extremely odd, anyhow, that your general performance would be that bad; the amoebas are one thing, but even your baseline is just suspiciously low. Is there anything else in the background that might be impacting it? Sometimes when I accidentally have two copies of AVWW open rather than one, I wind up with really choppy performance on the second one until I realize what I did.
I have heard from some other developers that the ATI drivers have been enormously buggy lately, but I don't know anything about it directly, not even the validity of that. Take that for what you will, that's just what I heard, and it was in reference to features that we specifically don't even use. So I don't know if that would even come into effect here.
Another thought: are you using vsync? It might be a good idea to try turning that off and see if that helps. You could also crank up the target FPS and see what happens. It really makes no sense that your performance would be anywhere near that. For the record, my mac laptop has a builtin ATI card of some sort and gets more like 60fps, with occasional dips a bit lower.