When Plasma on Wayland implementation comes in with per-display DPI, one thing that could be a great addition to KCM is resizing the presentation according to scaling factors that were applied per each display. Example: http://imgur.com/a/ZT8iS The laptop screen is QHD+ and has 13.3" diagonal. The external displays are 27" and have QHD resolution. In real life the externals are much bigger and laptop screen takes about a quarter of each external display. In the visualization the laptop screen overwhelms the externals.
I'll add that as a suggestion / idea to our new design, but I'll close this report so it doesn't show up in the list of defects, this makes bugreports easier to handle. Thanks for the input!