I totally missed that there's an xorg.conf in this system. I always went straight to xorg.conf.d and was puzzled to only find the default files there.
@systemd: I don't want to turn this into yet another systemd thread and my knowledge about init systems isn't deep enough to actually judge that matter. With the dependency situation in the long run I mean post-jessie. The quality of the software in the debian repos varies greatly and my concern is that some packages in the "outskirts" may not be tested enough against a 2nd non-default init system.