Man, I just finished the SP demo myself and the framerate is smooth. Those graphics are insane though, the mlaa really comes into it's own on certain edges, it's just unquestionable, really nice IQ. Of course, the graphics. I'm sure these are the best I've seen in a while and I've played quite a few of the lookers across varying platforms. I really like the motion blur implementation whilst running, and the effects in that demo level are really impressive, you really get the sense that the wind coupled with the snow are menacing, perhaps not to sev though, since he's just wearing a shirt, lol.
Can't wait for the new locales in the main game, playing this demo already says that it's going to be an intense ride from start to finish. I suspect, playing this on Elite wont be a walk in the park either, the Ai in KZ3 are pretty good even on normal difficulty.