Blurry font when connecting MBP Mid-2012, Retina to an HPes 27" display
How are you connecting to the display?

I've used screens in the past (a particular model dell springs to mind) which applied sharpening to the HDMI input by default, resulting in "crunchy-looking" type. That could be worth checking.

It's also fair to say that the MacOS doesn't always handle transitioning windows from retina to non-retina screens -- I've seen plenty of instances where the windows fonts in FCPX become extra-jagged until some magic combination of moving the window from screen to screen and/or rebooting the mac solves the issue. That's with a mix of screens, including the thunderbolt display.