Maybe our environment will help.
We have 45 exchange servers, with only about 10 in the same data center as our blackberry servers....corporate headquarters. Half of our blackberry users are on those 10 exchange servers, 1200. We have 2 BES servers servicing those 1200 users because ping times are less than 1ms. The other 1200 users are on 25 different exchange servers. Our average ping times to those servers is around 50ms. With that we can only put around 300 users on a BES before we start seeing problems. Our numbers have gone up due to grouping by exchange server, but that is still going on due to not every user having 4.0 on their blackberry.