I'm on the following spec...
i7-950
7970
8GB
W7
I was getting loads of stuttering, the Cat 15.11.1 driver helped some, but the new Crimson even more so. I also edited the AC.ini file to disable Bloom and PostFX. I added a Tesselation= line like AC Unity has, with it set to 0, just in case that would help as well.
I also dropped res to 1600x900 and put everything at Medium with FXAA and no Ambient Occlusion. I keep Vsync on though because I hate seeing the screen tear with it off. I use RTSS set to 57 FPS just to avoid any possible input lag.
The game runs a lot smoother now, but still hitches a bit here and there. The one hopeful thing is, Steam has listed the update file PS4 and Xone got since 11/25. So maybe soon we'll get the same patch, and at the top of it's release notes are stability and performance improvements.
Dropping to Medium isn't as much a visual difference as I worried it would be, and IMO, disabling Bloom and PostFX only made it look better. The lighting and faces are no longer overly glowy, and I don't see that thick bright haze at dusk that really washes out colors. Sunlit areas look equally good with no PostFX.
Disabling AO also makes nights and dark areas look darker. The only caveat to that is I compensate by bumping brightness one notch and adding about 5% more saturation on my TV to keep colors from washing out.
I can't believe games like Unity and Syndicate that boast such high end graphics use a Brightness slider with such few positions on the slider. I was hoping the AC.ini file would have a Brightness command with more scalable values, but like Unity, it has nothing at all for Brightness.
This is the kind of thing you can expect from a large dev/pub game maker that thinks of console first, with PC as an afterthought.