I have an external Ultra-HD monitor. I would like to use this 2nd screen in Coherence mode for both Mac (Retina-scaled) and Windows (Windows scaling). Windows screen resolution looks blurry and the text is crisp.
In FullScreen/Coherence mode a (Windows) virtual machine can not be given a monitor resolution equal to the native monitor resolution, but only the logical monitor resolution or twice the logical monitor resolution.
The virtual machine is not able to draw anything using native pixels/coordinates, only in logical coordinates.
Mac OS logic will always scale the drawn pixels by Windows from logical to native coordinates (hence the bad resolution) and this can not be bypassed.
When you are using Retina on 5k-display Mac. In this case the logical resolution will be 3008x1692 in Windows. It will be almost 3/4 of native Mac resolution.
This behavior repeats when you install Windows in Boot Camp. And this issue is related to Mac infrastructure, not the virtualization part.