Just found an interesting article about IMAP clients comparison:
http://www.hackvalue.nl/en/article/64/stresstesting_imap_clients
Personally I'm using claws-mail, with 4 mail accounts, one of which has more than 250000 messages.
Migrating to it from thunderbird was fairly easy, I've converted my
thunderbird filters to claws' format, using
http://www.claws-mail.org/tools/claws-mail-thunderbird-filters-convertor.tar.gz
(See http://www.claws-mail.org/tools.php)