I was able to get 60 Hz by simply enabling DP1.2 on the monitor, but then I had two virtual displays, which was quite annoying. The patches to make the whole stack properly support this sort of monitor are not quite there last I checked (a few weeks ago). Dave Airlie, who's responsible for most of the work on this at all levels (from the kernel through through the window manager) has things mostly working, but it's not ready for prime time. He ended up giving up on doing everything entirely in the kernel as he had originally tried, so now everything up the stack to the window manager needs patching. He has experimental support in GNOME. I may attempt stretching myself and seeing if I can get similar changes into kwin while I'm on winter break, but I've found 30 Hz to be entirely bearable; most of the time I don't even notice.