The verdict is that it works fine. The thing about root, though, is that on unlocked phones, "exploits" or "voodoo" aren't necessary. If you can flash custom recovery and boot it, i.e. twrp or cwm, then you can root your device much easier than all the other stuff. Tucs posted his link to it earlier in the thread. Process I used is as folllows:
o) Since I was on CM 10.1 I had to nandroid it and restore what I thought was MR2
o) MR2 was mislabeled and was in fact MR4. Yey me.
o) Used my House of RAZR_HD utility to perform an FXZ Keep Data operation for MR2 on top of MR4.
o) Flashed stock recovery from MR2 for good measure, even though the FXZ took care of that
o) Booted up stock recovery, flashed the update, got the bl_backup error as noted
o) Wiped cache as I always do on an update
o) Reboot system, let it run the android update script
o) Verified root was toast
o) Reboot to AP Fastboot
o) Flashed TWRP 126.96.36.199
o) IMMEDIATELY booted into recovery...DO NOT let the system boot else it'll replace recovery
o) Installed supersu flashable zip
o) Rebooted, verified my apps had root
o) Boot back into recovery, take nandroid