Yeah, it's a huge charlie foxtrot, no doubt about that. Keep in mind when looking at framerates that the 6850 benches ~40% higher than the 5770 and I've got an i5 running with a steep overclock and (probably) larger L2/L3 caches, unless you've got one of the Black Editions. Even I get stuttering at the beginning when everything starts shooting, and then it evens out. Massive Battle pretty much is a huge FSO benchmark, if you can even get 5 FPS in it, it usually means you're good to go for anything the engine can throw at you (except maybe Delenda Est before the optimized Karuna - brutal!).
Massive Battle is our personal Crysis.
Also yeah, I wonder how CCC isn't able to force morphological AA with FSO in fullscreen...it can do it in almost every single other game, which isn't to say that I particularly LIKE morphological AA...