I just purchased an unlocked 1520.3 and put it on a T-Mobile account. I got the usual 611 when I tried to use internet sharing.
I enrolled the phone in the M$ Dev program and started the 8.1 update. The phone settings were changed in the registration update step and I was then able to use internet sharing. Internet sharing continued to function after the 8.1 update and the latest maintenance update to the 12397 level.
While 8.1 is so much better than 8.0, there are issues which may make it difficult for anyone who needs to have the phone as their only one, survive the frustrations of problem solving and having it usable for daily use.