Whilst we wait for official news on Sony’s upgrade plans to Android 4.4 KitKat, xda-developer pabx has just released an AOSP Android 4.4 custom ROM for the Xperia Z. The ROM is surprisingly stable for those that have tried it, with some people reporting of issues with Google Calendar syncing. If you want to give it a go head over to xda for download links and further information.
Thanks Ben and Felix!