Finally Sony released Android KitKat update to their Xperia Z Ultra, Z1, Z1 Compact series and since then, i have been gathering more information about this update through their Forum and i saw LOTS of issues regarding this update, starting from quick battery drain, easily to get overheated, a little lag, speaker low/loud alternatively, and many others. Also, some new features found on Z2 are not backported to Z1 update as well, so that's quite disappointing for me.
Anyway, i decided to take the risk and last night, i reboot my computer to switch to Windows and connect my phone to Windows and ran Sony PC Companion to detect the update. It started to download the updates, but since it was 1 AM in the morning, i went to sleep. This morning, i plugged the phone again and it started to upgrade to KitKat. There was no problem during upgrading and after the reboot, the upgrade process continued and everything is still there. There are no data loss at all.
About battery drain, i also experience it myself, although i think it's only a matter of time before the stamina mode find your usage pattern and it will try to fix it by themselves. I tend to turn off the location, GPS, and only use the WiFI or data packets if it's necessary. So far, it's good.
One thing i wanted to try was the ART runtime which is a technology preview for the future Android release that will bring performance improvements in overall system/apps. Unfortunately, Sony seems to disable this feature on their firmware, as i can't find it on the developer mode. After browsing some sites, i found that Sony Album, their pre-installed application is not yet compatible with ART, so they disable it. I'm hoping that Sony will be able to fix it soon and release an update to bring ART runtime to it's users. You can check for list of apps that is reported to be working/not working with ART runtime on this site
I'm now a KitKat users :)