I have a Thinkpad Yoga 2 Pro (3200x1800 @ 13"), and often use an external monitor. My "solution" is to turn off HiDPI and rely on the zoom features in all of my apps. (terminal, emacs & browsers)
Browser windows on the laptop usually run at a zoom of 200%, whereas windows on the external display run at 90%.
Annoying, but the beautiful text is worth the bother.