Update: Xperia Blog has confirmed this update is, in fact, Android 4.3. I can confirm that T-Mobile does, in fact, suck at changelogs. The version has also been tweaked to 10.4.C.0.814.
It might not be the latest and greatest anymore, but the Sony Xperia Z on T-Mobile is still getting some update love today. That mysterious Android 4.3 update that was pulled a few months back still isn't back, apparently. Instead, you get some small tweaks.
T-Mobile will be pushing the OTA to devices throughout the month, but you can also use Sony's desktop companion software to manually install the update. It's pretty hefty, clocking in at 312MB. T-Mobile's docs don't list a bump to Android 4.3, even though 10.4.C.0.808 is more than large enough for that. It's still possible this is Android 4.3 and T-Mobile just sucks at changelogs.
The only new feature listed is the option to do a master reset using the hardware keys. The software tweaks include interface changes and the merging of several Sony apps. We'll see what happens when it starts hitting devices.