I can also confirm that DX11 is "broken". With 670 + 460 1GB as PhysX, all maxed, I got lot of stuttering ingame. I tried lot of combinations (for hours, stupid out of game settings and long intro
) and these are my observations:
1) Dx11 + Physx High - stutterfest (even if I turn down other settings), GPU usage on both cards <50%
2) Dx11 + Physx Medium to Off - Better but still some stuttering
3) Dx9 + Physx High - Finally playable without problems, but in benchmark scene with Iceman I got better Fps with 460 disabled and PhysX run on 670.
This leads me to thinking the is some CPU limitation going on since I have "lowly" i5-760 @ 4.1GHz and with dedicated PhysX card there is probably larger driver overhead and thus more work for CPU.
Overall DX11 feels totally unoptimized, because with Dx9 I get steady 60fps, and if there is some rare hitch it goes to +-50, but Dx11 is constantly dropping to teens. Maybe with IB@5GHz and running of raid0 SSDs it would be better but I'm not entirelly convinced. Will test in a month or so when I upgrade mobo+CPU.
So my final settings are: Dx9, everything maxed, 8xAA and High Physx and I'm enjoying smooth gameplay using only 670.