Test it? If you already have that rig available, just do a test recording for the settings you would want and see how far you can push the quality setting for x264 before you reach an encoding limit. There will be a little bit of overhead based on how complex your scene will be, but that will give you a rough idea.
Ballpark guess? Probably able to do 1080p60 fast/medium, 720p60 medium/slow. Pure guess though, haven't had a chance to play with that chip.