No data migration. Use POP, download your emails then install M3 :-). The type 3 should be a configurable size. For instance I might only want mails above 4k in the store and the rest cached. that'd give me a good compromise.
I have modified the Entity Mailbox to hold a serialized copy of the mail body. I think there is enough of the store in place now that the new mailbox implementation could be integrated. Most of the changes outstanding for the store will be behind the Store interfaces (e.g. configurable minimum stored message size).