Sony Mobile’s Xperia X Concept team has released a new firmware update (38.3.1.A.0.54), which adds X-Reality for video, as well as improving the backlight brightness control. X-Reality has featured in stock Xperia software for a while now, but had not been included in Concept firmware up until now.
There are various other improvements and bug fixes, including a fix for the bug when restarting the phone in low battery mode. VoLTE was temporarily dropped in the last update, but now it is back (remember only Germany, Spain and UK networks are currently supported). Sony has also updated the Google security patches to 1 March 2017.
Thanks Josef and Najodleglejszy!