@30knees
No, I don't use phplist with tb. I doubt phplist will be a good tool for transactional emails. However I believe you could export and import users and newsletter preferences. I remember there was also a php script you could call to add members to phplist, that I had used elsewhere many years ago.
For my original point of queuing emails for orders from tb however, I have now added delay to the configuration in postfix, since I run my own mail server. I believe this should allow creation of the email quickly but deliver it leisurely, not holding up the order completion. However, I have hit a snag in that presta/tb don't seem to work well with SMTP email, so I am sticking to phpmail until I can sort this out.