Unfortunately, to be able to run xorg in unprivileged mode, it needs to run inside a session. So, right now the handicap of running xorg as user service is that it must be run with root privileges [...snip...] It seems likely that xorg could be modified [...snip...], and then it could run unprivileged from a user service outside a session.
https://wiki.archlinux.org/index.php/Systemd/User#Xorg_as_a_...