[M4tt0]

(1) VGA Modeline: Modeline "1360x768"x0.0 85.50 1360 1430 1542 1792 768 771 777 795 +hsync +vsync (47.7 kHz eP)
taken from Xorg.0.log (with HDMI and MiniDP -> VGA connections attached):
http://paste.ubuntu.com/8371052/

(2) Updated xorg.conf:
http://paste.ubuntu.com/8371101/

(3) MiniDP disconnected -> Reboot -> Black screen... :-(

(4) Xorg.0.log now: Modeline "1360x768@60"x60.0 85.50 1360 1430 1542 1792 768 771 777 795 +hsync +vsync (47.7 kHz UP)
http://paste.ubuntu.com/8371130/
There is a difference here as the line ends with (47.7 kHz UP) now whereas it was (.. eP) before, but I failed to figure out what that means and whether it has implications...

(5) Reverting back to 720p@50, which - thanks to this build - still gives a picture that is way better than anything I have had before...

Thanks again for your help, fritsch!