I also own a Sony Tablet Z which I love. I have tried many, many other tablets, including Samsung and Apple, and I prefer the Sony Tablet Z, I would just like to have a bit more power with the sound (easily fixable with a Bluetooth speaker).
I also owned a Blackberry Playbook, which was so underestimated as it is a very good device with the best audio of all the devices I ever tried, too bad the developers did not get into the device and too bad blackberry did not convert it to BB10.
I have read that Kitkat is coming soon:
"Sony is in the process of Android updates with the manufacturer having rolled out Android 4.3 Jelly Bean onto some of its higher-end devices recently. Some of these devices, which include the Xperia Z, Xperia ZL, Xperia Tablet Z, Xperia Z Ultra and Xperia Z1 have been confirmed by Sony for Android 4.4 updates. The Japanese manufacturer; however, has not revealed any specific rollout dates."
I have read that it should be available by end of February!
I also found another forum group
Xperia Tablet Z General - xda-developers , lots of the content is for developers, people that root their devices etc, but you can find interesting stuff about your device in there.
I personally keep my device stock, but not because I am against rooting, it is just because I am afraid of damaging my device as I am a bit of electronically challenged! I am just a good user.
Hope this helps.