The Exchange team published a nice article on the different performance characteristics between Windows Server 2008 SP2 and Windows Server 2008 R2 when running Outlook Anywhere against Exchange 2010. It seems Exchange 2010 Client Access Server can handle more than double the amount of clients when bound by CPU resources when running on Windows Server 2008 R2 instead of Windows Server 2008 SP2.
The team also expects big improvements in Exchange Server 2007 SP3, schedulded for 2nd half of 2010, because these improvements are caused by differences in the underlying Operating System, which means it will affect RPC/HTTP performance as well.
Another thing to notice is that above 3.000 users the MHz per Outlook Anywhere user for non-CAS roles, when used in combination with other server roles, are nearly flatline. Where Client Access Server shows a linear graph (steepness depending on Operating System used), Mailbox Servers show an average of 0.6 MHz per user and Hub Transport Servers around 0.25 per user. During testing, LoadGen’s OutlookAnywhere_100 profile was used, formerly known as Heavy User with 20 sends and 80 receives per day.
Be advised tests were performed with Basic Authentication; NTLM drops the number of users by only a small 2%.
You can find the article here.