Hi all.
Its not an original idea (I don't have any). But I read somewhere that ARMA 2 enables Vertical Sync ( or "wait for refresh") by default and there's no ingame option to disable it.
I forced it to be "Always Off" in ATI's CCC and I'm now avg. 30-35 fps in "Manhattan" on mostly high settings.
Also I find that,since then, adjusting in-game settings (useless before) now has more of an impact on ARMA 2's performance. Draw distance, resolution,AA, etc.
I also disabled Catalyst AI which might have helped as well.
That's been my experience anyway. Hope it helps.
Cheers
Ceedub