I have the same Problem. I have a 9800gx2 and i only can play the game on normal settings. Because the trees and bushes kill my FPS.
If i look with a scope throu a bush in to the sun, my frames going from 60 fps to 15 fps.
Also if i stand 2 meter from a bush and zoom in to it or not. Also 15fps.
Its not playable. I had the same issue with ArmA1.
After the newest nvidia patch 185.58 that was gone. But in ArmA2 it is now back.
Also i have flickering if SLI is anabled in ArmA1 bevore that driver. In ArmA2 i have only flickering on loading screen (like in the disco) ^^.
I think the problem is something with that HDR Effect.
I hope they will fix that quick, because it realy suxs. :-(