I am trying to edit 2560x1440 at 60 frames per second, but using h.264 level 5.1 limits the output to 2560x1440 at 50 frames per second.
Looks like you will need Level 5.2, which PP simply can't do at this point. You may have to export out a lossless Intermediate and do your encoding in another program.
This wikipedia link mentions that Adobe CS6 will support the new Sony XAVC codec. http://en.wikipedia.org/wiki/XAVC
When will it be supported?
The new version, hopefully coming in early May.
x264pro has 5.2 support.