I'm sticking with KMail because it's the only mail client that allows me to handle a thousand mails a day, quite apart of the many thousands of spam mails it also has to handle.
But some time soon I'll have to spend a day or two, three cleaning up and converting. I'm sure it'll work fine when the conversion is done, and I don't think it's the database that's the problem, at least not for me. I'm sure the database will be great, when fed a few thousand mails a day until it's 6G again.