[Dovecot] ldap performance?

We're looking at replacing our mysql auth and userdb, with a set of Red Hat Directory servers, and I see dovecot supports either authentication binds, or "password lookups":

http://wiki.dovecot.org/AuthDatabase/LDAP

I'm curious if anybody can give any insight as to what kind of performance to expect from authentication binds ? How many POP3s/second should we expect from it ?

Our directory servers will be 2 blades with 4 cores of 2GHz each, and we expect to keep the full directory in memory on both servers.

-jf