Bearman:
I believe you, but I don't believe T-Mobile. Remember all the useless, contradictory, and misinformation about WiFi Calling on the 8X? I can only hope that the tech you spoke to was as ill informed as some of the others.
I understand the basic idea of getting GDR3 by registering as a developer, but do you know what happens if I've done that, installed GDR3, and then T-Mobile pushes out the update? If it would cleanly overwrite the first install I'd consider doing it.