Wait, even after you
cvt the resolution and refresh rate you wanted and added its modeline?
I found that in Ubuntu MATE 16.04 I couldn't get the resolution i wanted for an HDTV attached to my mother's laptop (my family use it as a media centre) but in 16.10 xrandr seems to just work. This is even from live.
Whatever version xrandr / X.Org is for 16.10, it needs to be backported to 16.04 for me to consider using the LTS version again.