They are "the code doesn't exist" issues. Wayland will fix it; and there's a patchset nearly ready to mege
There is supposedly a way using xrandr --scale. Effectively you set the non high DPI monitor to be twice the size. Set KDE to render everything twice the size using the scale thing then you use --scale to make X downsample the one monitor when rendering. I've never seen it work, as it requires some Ubuntu-only Xorg patches, and it will just bring you some other problems with apps that can't scale themselves.
