Simply change this line in your build.prop file.
change it to:
Reboot your device.
If it doesn't work right away you simply go to YouTube App Info and press Clear cache and Clear data. Then open the YouTube app again and enjoy 1080p!
Hope this helps you guys and big thanks to twizzlebizzle21 for discovering this. Here is the original post on reddit http://www.reddit.com/r/Android/comm...e_by_changing/
Works perfectly on the latest build of Mahdi rom for me.