I opened a bug against gnome
in the meantime. I was able to get rid of tearing on the secondary screen altogether with
$ __GL_SYNC_DISPLAY_DEVICE=DFP-2 gnome-shell --replace --display=:0
The only issue remaining is the "static tear line" on the laptop screen when it was selected as device driver should sync to. Basically, windowed mplayer looks OK but in full screen there is a static (as in not moving) tear line close to the top of the screen (with xv) plus some small artifacts appearing in the bottom part (vdpau and gl). Can it be caused by the fact that, according to nvidia-settings, external screen is 60.00 Hz while the internal one is 59.93 Hz? Could someone from nvidia please comment on this?