With over 250,000 Exchange mailboxes and a multitude of F5 products usage patterns have indeed changed with the changing Exchange versions.
Exchange 2003 is hardly the best comparison to Exchange 2010, but when dealing with 10,000 mailboxes at your largest site a 6900 model is complete overkill even if loading all of the bells and whistles. We maintain over 600k concurrent Exchange connections mostly RPC/HTTPS in a 35 node CAS Array on a 6900, SSL Offloading, irules, etc and the the unit is bored.
I would indeed engage your local F5 rep but IMHO a 1600 pair for your small site and a 3600 pair for your large site will likely give you the performance you need.