Directx 11 can make a big difference. Just look at the heaven benchmark (unigine)
Or... you can look at a game built originally for DX9/10 with DX11 added onto it (like BFBC2).
It's foolish to compare Crysis 2, which is a game made primarily with DX9 in mind with a benchmark made to showcase (and let's face it - to advertise) the hardware tessellation of DX11.
Slapping a DX11 pipeline onto an already made game isn't going to somehow magically make things better. If crytek had developed crysis 2 FROM THE START with DX11 in mind and used the pipeline to take advantage of all of it's features then maybe it would've made a "big difference." IMO gluing it on to the final product as a patch isn't going to be a giant game changer by any degree.
Also @yakkern - let's not attack each other based on race, yeah? I can see where you're coming from, but making every single glassy object in the damn game to have realtime reflections is clearly not going to help. I don't think I was very clear about that in my original post.