Interesting performance comparison report!

Principled Technologies tested Harpertown and Nehalem servers running various Windows Server operating systems to show the benefits of processer and of operating system improvements on simulated customer workloads.  Windows Server 2008 R2 delivers over 37% more capacity on these tests on the same Nehalem-based servers.

http://www.principledtechnologies.com/clients/reports/Microsoft/HyperVR2_0709.pdf