Microsoft has published a performance analysis of IIS 7.0 on Windows Server 2008, versus IIS 6.0 on Windows Server 2003. In summary, the results show that IIS 7.0 is more efficient, and faster when running on the same hardware! You would be crazy not to upgrade!
The full report is available here:
Microsoft.com Operations Performance Analysis of IIS 7.0/Windows Server 2008