I've found a few settings that are key to maintaining a high fps on a medium-range system. They are
- SSAO
SSAO -> HDAO takes 7-8 fps right off. SSAO looks like shit. Just disable it.
- Shadows
Lowering it to medium from high did wonders.
- Post-process FX
Same as shadows, medium works MUCH better than high.
- MSAA
Holy shit what a performance hog. Even 2xMSAA shaves ~10 fps off the top... wow.
- Surprisingly, putting "Alpha to Coverage" (foliage AA basically) from "off" to "enhanced" gives me only a 2-3 fps hit! It does A LOT for the image quality, since it greatly reduces shimmering in trees and foliage. So I recommend cranking that one up to maxium. I can't stress enough how much better it makes the game look, especially if you don't enable MSAA (which you probably shouldn't with a mid-range system).
EDIT! Since I wrote this, I've tried turning AtC off, and I have to admit I may have exaggerated the effect. I left it off, and instead turned on HDAO. HDAO has a huge impact on the depth of any scene, since it adds lots of subtle shadowing to the environments. The trade-off was worth it.
Setting everything else to high, with DX11, triple-buffered Vsync through D3DOverrider and 16x high-quality AF through nVidia Inspector, I'm able to maintain 50-60 fps just running around a bit in daylight with the following system:
2500k @ 4.5 GHz
GTX 560Ti @ stock with beta drivers
8 GB RAM
I'll be playing for real when I get home, and posting some more performance impressions then.