I can confirm that fast.tmobile.com works great on my Nexus 5 with SFOS, but inconsistently on my Xperia X with SFOS.

My observations:

Nexus 5 - I get advertised LTE data speeds on fast. For MMS, I just switch to epc to get it to work.

Xperia X - I don't get data speeds approaching what I got using SFOS on the Nexus 5. In fact, on the Xperia X, performance on fast actually seems worse than on epc. In any event, I'm using epc: I'm not getting LTE speeds, but at least performance is reasonable and consistent.