How can I change DPI on OS X?
Not sure if this will help some people, but I figured this out for myself

I have a 27" LG WQHD (2560x1440) display connected via HDMI to my 2014 13" Macbook Pro Retina.


Enable Mirror Displays





On the external monitor, select Optimize for: Built-in Retina Display




The final result isn't perfect, but it makes my 27" external display look like a beautiful retina display.


  Two downfalls:



the macbook has to be open in order for the external to continue running in HiDPI mode.
the external monitor has an aspect ratio of 16:9 but the laptop is 16:10. There are thin vertical black bars on the left/right of my external monitor.