Well, it depends on what is implemented in DX11. DX11, if optimized right without adding ANY features should be faster than Dx9. So it all depends - if they add tensel and one cranks it up = well any single card will crash to its knees. So for all we know we might get better AA, better water, same FPS (or better).
Well, I have it on my shelf until (1) all the patch/code issues are fixed and (2) dx11 comes out. IT will be a nice summer game - just hope some people online are still playing.
I just don't see myself playing the single player more than once. So why not wait for the "way it should be played" with DX11 and experience all teh glory. Need to start (and finish) AC2: Brotherhood