Hm. I could go and install Dom2 on my second PC, which has a GeForce. But I'm too lazy to actually try it.
Btw. FPS don't change when going from Windowed to Fullscreen. (I usually play Windowed)
I must admit, I'm not really concerned. 20 FPS on the battlescreen. So what. It's not like we're playing some clickfest here.
And I'm fast-forwarding through the battle most of the time anyway.
Now, under 10 fps on the higher settings, that is another story. I don't really see how merely increasing visibility range on the trees should cost so many FPS...