To clarify, there's no MetroPCS version of the M8. My wife wanted an M8 and I got an unlocked Verizon M8 GSM. It's a Verizon phone, but it works under any GSM carrier. If you're wondering why on earth I didn't just get a T-Mobile M8, its because I got a real good deal on this one and my research suggested I wouldn't have any issues.
Anyway, I got the T-Mobile flashable zip off this thread:
http://forum.xda-developers.com/showthread.php?t=2736921
I also followed the directions there to create a back up of the stock radio in adb. I tried flashing the backup the way that's shown there. If you check, not only do they give you a flashable zip, but also the option to download the IMG file itself and instructions of how to flash it through adb.
So this is how it went down: I downloaded the T-Mobile radio, flashed it in TWRP since it was simpler, it flashed successfully but didn't work for my phone. I mean, in hindsight, of course it didn't. I flashed T-Mobile firmware in a Verizon phone... I thought it wouldn't be a big deal to return to stock radio firmware so I gave it a try. Before this I had spent countless hours attempting to solve the loss of data and SMS issue, I did this thinking "won't hurt to try"
Anyhow, I deduced that my backup IMG could be flashed just as the IMG they provide in that thread. I followed the instructions and that failed. Then I found a guide on flashing radios that stated the IMG can indeed be flashed through fastboot. However, it won't let me. I do believe it is possible since both adb and fastboot try to flash it, but fail due to lack of permission.
After this clarification, should I still follow your last advice, or can you think of something else? Do you happen to know how to turn my backup radio firmware into a flashable zip as the ones provided in that thread?
Thanks for your help.
EDIT
I forgot to mention. Last morning, before any of this happened, the phone's data connection stopped working all together. LTE icon replaced by an exclamation mark. Which didn't make any sense at all since the APN were properly set up. That's what convinced me to try flashing the t-mobile radio firmware