So does Maildir; reusing it between Mutt and GNUs or Claws Mail should be a child's play.

Once you have tar to preserve perms just in case, your are done.

Mbox on big mailboxes it's hell, anyone can understand that linear parsing will be slow as hell. It's like looking up a word file in a dictionary word by word from A to Z instead of directly heading to the first word letter...