Here are some benchmarks posted around the web.
http://www.maximumpc.com/article/reviews/windows_7_review?page=0,0
http://www.testfreaks.com/blog/information/windows-xp-vs-vista-vs-7/
http://www.paulspoerry.com/2009/08/13/windows-7-benchmarks-xp-vs-vista-vs-7/
There are more I can post. Most all of them show very close results between
XPSP3 and Win7 (some were pre RTM tests.)
There are great performance upgrades in networking and processing speed with
Win7. Same hardware, etc.
My testing shows similar results (perf on IIS, SQL, etc.) between Server 03
R2 and Server08.
Basically, I'll take all the new features in Win7 and Server08 and there is
no noticeable decline in performance across the board.
What I would really like to see is a new benchmark comparison with all the
latest WHQL approved drivers for video, controllers, mobo, etc.
It is my assertion that Win7 will blow XPSP3 away in those instances.