Fonts on external display (4k TV) look awful, sub pixel font smoothing gone haywire?
I have/had a similar problem with my MacBook Pro and now my newer MBPr with an Asus 27" monitor. I ended up finding a discussion about it and a script in a forum for Dell Monitors.  The script took the monitors EDID information directly and created a display profile for it that overrode what OS X was trying to automatically configure.

Here is the script on github along with a link to the original topic. https://gist.github.com/ejdyksen/8302862

This worked for me to make my fonts readable again on my Asus VE27q.