I remember playing Arkham City with a 6850 and PhenomIIx4 3.7Ghz, PhysX High and everything on. Sure, there were some laggy instances but after the first or the second patch(the one that fixed Dx11) I can't remember seeing it crawl. I didn't get 60fps everywhere for sure, but never had any noticeable slowdown, except the split second when loading a new zone in the main world. That surely attest to Rocksteady optimization.
Now, with a 7950 in place and the Phenom cranked up to 4.0ghz(I must stop going to live gigs, I need a new cpu
), we'll see how it behaves when PhysX is on. All in all, this is still a UE3 game