Versions:
* plasma-framework: 5.9.0-0ubuntu1
* plasma-desktop: 4:5.3.0-0ubuntu1~ubuntu15.04~ppa1

My environment:
* 2x 2560x1600 @ 100 dpi side-by-side dual-head setup
* 1x DualLink-DVI directly from graphics card
* 1x DisplayPort via active adapter to DualLink-DVI (because monitors only have DVI)

The xorg.conf contains a section as recommend/suggested/proposed by the following page:
* https://community.kde.org/KDE/High-dpi_issues#User_settable.2C_overall_DPI

With the following contents for the DisplayPort connector (which are based on EDID information from the DVI-connected monitor - both are exact same model):

Option          "UseEdidDpi" "false"
DisplaySize     646 406

However, KDE/Plasma does not pick up that information, and xdpyinfo shows:
  resolution:    201x100 dots per inch

For a workaround I introduced an Xsession script with the following contents:
[ -x /usr/bin/xrandr ] && /usr/bin/xrandr --dpi 100

This fixes an icon scaling problem within the application launcher, activity manager and several other issues (e.g. resulting DPI of scanned artifacts).

However, although I now have the correct DPI set, icons and text inside the default panel are way too small so that e.g. the text of windows is completely unreadable.

Reproducible: Always