Huge thanks to the OP, hoodred !!!
I've been looking for a solution to T-Mobile's willful negligence of us Xperia Z (C6606) customers.
Now I wanted to share my experience and I hope it helps others who bought this phone thinking T-MO would support us.
First off, I'm no expert so I'm not responsible if you brick your phone or something else goes wrong.
I'm just passing on what MY experience was and I hope you can benefit from it.
Second, I'm no expert, as I've stated, so don't expect any tech support from me. XDA is a vast resource, but it takes some work to get the results you want.
All I can tell you is to use the grey matter between your ears and research (a lot) before you proceed. Make sure you know what you're doing.
Watch some of those youtube vids that show you step by step how to flash a new ROM.
I can't (won't) guarantee that this will work, all I can tell you is that it worked for me.
*** PROCEED AT YOUR OWN RISK ***
I was able to successfully load this (the OP's) firmware onto my T-Mobile C6606 which has a locked bootloader.
Yes, I made all the checks ahead of time, I checked the service menu and it does indeed say "Bootloader unlock allowed - No"
So I thought I was boned... But I tried installing Nut's XZ Dual Recovery and that worked.
Please note that I had already rooted with Towelroot.
Once I was at this point, I backed up all my files from the internal storage and made sure everything was synced with google.
Now I was committed; I did a factory reset to clean house, then rebooted into CWM.
First off I performed a backup in CWM, it took about 8 minutes.
Then, in CWM, I cleaned cache, cleaned dalvic, then did the "prep for new ROM option."
Once that completed, I had CWM install from the zip file from the OP, not the FTF (although I suppose that's another way to go...)
Flashing the new ROM was pretty quick and I just rebooted after.
First boot with the new ROM took a little longer, but it wasn't extremely long, I don't think it took more than 5-10 minutes.
Once it got to the new phone setup, I knew I was home free.
I entered my WiFi settings and google account and it just finished updating everything else on its own.
SuperSU was pre-installed so I didn't have any problem with TiBackup or any other rooted apps.
I hope this helps some of you T-Mobile people with one of these phones. I was going nuts waiting for them to release an update but that's just crazy.
I can't tell you how happy I am to finally be on LP. It's on 5.0.1 right now and I've experienced no real issues.
The phone got moderately hot due to downloading so much data over the WiFi and all the processing and setup but otherwise, it's been excellent.
The phone now says that it's a C6616 everything seems to work correctly:
Mobile Bravia Engine, WiFi, Bluetooth, GPS, mobile data (I'll check APN's today just to verify settings...), camera, etc. I haven't come across any faults, but I'll update if I do.
Big thanks to Sony for keeping their word and supporting their customers, Great job, Sony!
Thanks again to hoodred !!! You Rock!