Your disk storage requirements are not necessarily going to change
unless of course you're in a hosting environment and haven't mentioned
that part.
Disk I/O will be less depending on whether or not your users are going
to be doing a send/receive every minute or so or, in the case of 5.5
users, hitting F5 for no good reason.
The sizing guidelines you will see assume mapi connections and assume
a certain level of concurrency with users but POP & OWA are different
beasts.

15k users on one server, probably, but not in a corporate environment
with large mailboxes and high concurrency. A hoster  (Oliver may jump
in here) may well run servers in the high thousand users but they will
be behind slower bandwidth links and that affects the I/O profile of
client access.