On Wed, 2009-05-27 at 14:44 +1000, James Brown wrote:

I am migrating our mail server from Courier-IMAP to Dovecot. We
currently use maildir storage format.

What are the advantages and disadvantages of Dovecot's dbox format
over maildir?

Speed. Nothing else. Maildir can handle errors better, because if index files get lost/break you don't lose any state. With dbox you lose message flags.

Does anyone have any performance benchmarks for both? What do people
use to test performance?

There is nothing that can benchmark real world usage. http://imapwiki.org/Benchmarking explains some of the reasons.