There's an interesting set of test results over at the Microsoft Exchange Team Blog:

We recently compared the performance of the Exchange 2010 Client Access role supporting Outlook Anywhere users on both Windows 2008 SP2 and Windows 2008 R2, and found that the improvements the Windows Team has made in R2 more than doubles the number of concurrent users a given server can support, assuming CPU is the limiting resource.

Did you catch that? By upgrading to Windows Server 2008 R2, your server can support more than twice as many concurrent Outlook Anywhere users.

That, friends, is what we call significant ROI. It's also significant in that Windows has a reputation for getting slower with each release, but here's hard proof that at least where Server R2 is concerned, you're likely to reap some performance benefits on existing hardware--possibly considerable benefits.

Obviously there's no guarantee you'll witness these kinds of improvements across the board, but it's something to consider while you weigh your migration options. Be sure to read the full post for all the test specifics and results.