Hmmm... Martin, if you still have a Xorg.log it means you have a really old installation,
That was true for versions under 1.16.0-3 as evidenced by the version comparison, but it is not true anymore, Xserver 1.17 dumps its logs to syslog. And syslog is trapped by journalctl.
Errrrrrrrrrrrrrrrrr No it does not this system is fully up to date .
I use startx to start the graphical display nothing in journalctl it is in /var/log/Xorg.0.log ....
Sorry to disapoint and all that ..
From what I've read, it's *only* GDM that "hacks" the X server to run so that it redirects the logs to the journal. Otherwise, it'll go to the /var/log/Xorg.0.log file as ever.
ps. on my two - always up-to-date - Arch installations, using lightdm and sddm X logs to /var/log/Xorg.0.log -- damjan