You should be running it at DX11.
With C2D systems, there is a lot of talk about dropping down to older DX versions to save some abuse on the CPU.
Granted, I have no idea why dropping from DX10 to DX9 on a GPU that is not exactly being taxed anyhow would bump framerate, but I've heard it mentioned a lot. For some reason some fixes seem more graphical in nature but the game's engine simply taxes the shit out of CPUs. It basically seems like regardless of clockspeed and chip type, if it's dual-core, you can't get maximum quality at playable framerates, no matter how awesome the GPU.
Looking at graphics quality and following the framerate while tweaking my own settings, it definitely appears my GTX285 could easily play this game at max quality settings, as long as I had more than two cores (C2Q at good clockspeed, AMD tri-core, Intel i5/i7, etc).
I've been thinking about trying this DX9 trick, it's the last framerate-boosting trick I've read everywhere (everything else essentially worked, be it maybe only a few frames/sec more, or 10+) that I haven't actually employed on this end.
There is still a fair amount of lag even on very low-ping servers (at least, low-ping in the server browser... 25ms on the browser, in game close to 100 most often, but still have seen below 50 in-game and lag is still there)... but if I could get a more playable framerate without going to ugly as shit settings, I'd quite like to reduce the visual problems to network lag instead of sub-par FPS
and lag.