there are some optimizations and changes to the 7 system architectures over Vista, so it should give better performance than Vista, and in some cases better performance then XP. for other cases less than XP, but should still be better than 7.
Vista had some major architecture changes to a lot of stuff. They've had a good period of time since Vista RTM to improve and streamline those, really tightening them up to be killer. I'll be keeping Vista on my notebook for a while, because of some problems with Beta and RC (namely that it didn't turn on any hardware every other boot) and because of my development environment and how deep into the project I am currently, but I'll be using my extra partition for the 7 house party and I'll do some direct benchmark comparisons before I format it again