On t-mobile running 4.3 ROM, and have working 3g/4g/LTE. What makes you say you will lose 4G? All I had to do was load the correct t-mobile APN's. Unless you know something I don't and the "H+" over my bars is a false reading, but this is the first I've heard you will lose 4g. Either way I couldn't be happier and my speeds/coverage are quite good considering where I live.
I rooted and flashed stock ROM the day I got it, and quit paying attention to what t-mobiles plans were for releasing new versions since the international gets it first and the devs here are all over updating stock ROMs. Unfortunately, carriers are typically behind international versions so if you're not going to root and flash a stock ROM then you'll probably be waiting a while for 4.3/4.4.
The loss of wifi calling does not bother me either. I was little disappointed the bootloader can't be unlocked, but honestly I'm really enjoying staying stock and Sony's UI is not overdone. Even if I could unlock, I probably wouldn't at this point.