We have started our migration today successfully migrating 6000 accounts today from our qmail-ldap platform to Zimbra. We have not threaded our migration script yet, but unthreaded it migrates 1000 accounts / hour. We never used imapsync (too slow) but rather used external data source to pop the (non imap) mail to the Zimbra side.

Will keep updating here with details and look forward to any questions.