Unfortunately you will never find yourself in a situation to actually use a mixed dpi X11 setup (you lose your homogeneous desktop) and Wayland is better at spoofing it (for whatever reason fractional scaling works better in Wayland).
http://wok.oblomov.eu/tecnologia/mixed-dpi-x11/
My favorite quote from that writeup.
"If you think this idea is a bit stupid, shed a tear for the future of the display servers: this same mechanism is essentially how Wayland compositors —Wayland being the purported future replacement for X— cope with mixed-DPI setups."