For anyone interested: I've been using a large 144Hz 2560x1440 monitor next to a pair of 60Hz 1920x1080 ones with only a touch of xrandr (one line per screen) to make the positioning comfortable for me (just as a matter of preference, due to monitor height differences).
Idunno when this person last checked (and seemingly then set a permanent opinion, as many do for some reason) but getting X to handle differing refresh rates in multi-monitor setups is trivial now, unless I'm missing something.
Hey, you don't know that there aren't UFOs out there with amazing chairs in them! That's why we have to identify them: to find out where the best chairs are flying in from!