IOW, dovecot would see an SSL connection too.
Hmm, yes. I took it to mean that the 'encrypt' of
encrypt(compress(imap stream))
was the "extra layer". But, I think your interpretation is more easily arrived at, and if it's what Mark meant, you're absolutely right that the tunnel won't help.
A compressed SSH tunnel to regular, non-SSL IMAP should work to reduce traffic, though.
Absolutely. And it'll even be considered 'secure' since local connections are secure.
OTOH, if you're going to the trouble to use ssh anyway, can Thunderbird do something like a "connect command"? I use that in evolution, and mine looks like something like this: ssh -C mailserver '/usr/sbin/dovecot --exec-mail imap'
where 'mailserver' really is an alias in my .ssh/config
johannes