http://dovecot.org/patches/1.1/maildir-dirty-syncs.diff
This patch adds a new maildir_very_dirty_syncs setting. If set to "yes", Dovecot assumes it's the only one changing the cur/ directory (so other MDAs can add mails to new/ without problems). This makes it possible to avoid rescanning the cur/ directory all the time looking for filenames. It also looks like (in stress testing) it still scans the cur/ directory unneededly in some situations, but it shouldn't be common. Maybe I'll look into it some day to see why.
I'd be interested in hearing how large performance benefits it gives, if any. If they're big I could consider adding it to v1.1.12. If they're pretty much irrelevant I probably won't even bother adding the code to v1.2. I guess you mainly see the performance benefits with large/huge mailboxes.