Brian -  We have migrated our smartermail servers several times first time we had around 230GB of data on each server to move  and the second time about 450GB for each server.  Stopping the source server and copying the files to the destination is way too slow to do in a maintenance window.  We used a fast file copy/syncing program that we would start running a week or two prior to the migration.  We would run it the first day and it would take a good amount of time.  Then we would run the sync every few days and even daily.  Then the night of the big move we would stop the source servers run the sync which would only take 20 to 60 minutes depending on if we ran the sync a few hours prior.  Then fire up the new server and we were good to go.  We only had our existing licenses and did not try to activate the license on the new server until we had all of the data over and fired up the new server on the migration night.  Hope that helps.

We have used FastCopy - doesn't look like much but it can copy things very quickly.  http://ipmsg.org/tools/fastcopy.html.en

We use ip addresses instead of server names.   We also have had success using InstantSync FTP http://sitedesigner.com/instantsync.htm also.  We use this for migrating customers from one server to another.