[Dovecot] proxy performance

Can anyone speak to experience using Dovecot proxy to frontend for multiple back-end servers in a "large" environment?

We have 50,000+ mailboxes spread across 10 POP/IMAP servers. We'd like to be able to put in a front-end setup on our load-balancer of 4 dual-CPU Sun systems. I am a bit concerned about performance issues although from what I have read the proxy can be set not to fork, which improves performance a lot. Still concerned about whether SSL loading will be an issue.

We'd like to use LDAP to look up the reference to which mail-store a user is on. I see that the Wiki refers to SQL. The mailing list indicates passwd-file also works. Not clear about LDAP.