[Dovecot] Dovecot performance under high load (vs. Courier)

Hi,

We are building a new system that will support a large number of users (high volume, high concurrent usage, etc).  We have played with Dovecot, but in most serious applications we have traditionally used Courier IMAP.  It's my (lay) understanding that with indexing and perhaps other things in Dovecot, it might perform better than Courier in larger environments like this.  Am I correct or is it less clear-cut?

Any tips on making the migration (not migrating an existing system, I mean migrating our paradigm - things to consider, things to watch out for)?

TIA