A mediumweight Messenger Express/Communications Express user base is similar to Mediumweight IMAP. This user base has the same message size mix as Mediumweight IMAP, Lightweight IMAP, and Heavyweight POP. And, the message delivery rates are the same as Mediumweight IMAP users.
It is likely that you will have more than one type of user base in your organization, particularly if you offer more than one client access option. Once you identify your user bases from these categories, you will test them with your usage profile and with a load simulator, described in Using a Messaging Server Load Simulator.