Testflight In App Purchases, sandbox user still needed after passing Beta Review? If our app has passed beta review, meaning we can distribute via TestFlight to 1000 external users, do we still need to setup test user accounts for in app purchases? 

Seems like that would be a huge burden, not only for developers (I guess people could share test user account logins, but that could screw up the testing !!!), but also for testers (testers have to logout of the app store on their phone, then log back in with test user account).
No. According to this article, sandbox accounts are automatically created for TestFlight users. Anyway I haven't been able to find this in the official Apple documentation. If anybody finds it it would be great to have an official reference.