You must be a whale if you are already 56 level hehe.
I can't see any exp raise in rewards from PvP, cause I am basically getting the same exact things on my alt-account that was level 10 now close to 20. Same looking mostly random exp, mojo and cash. Heck in last 50 battles I did before I stopped working on the alt-account - I was getting more exp on average there.
Also I insist that there is really an RNG component - like lest say you have a RNG based base value that then is multiplied by the rank/level difference/stuff. Cause like I said in my previous post - I got different exp/cash and even mojo IIRC on multiple occasions when I got the same enemy on the list of 3 and fought them few times in a row. Thou not sure if they were NPC with what you say random rank or something.
Ages ago the NPC were easy to find cause they where the only enemies with level above cap, but lately I didn't spot a single enemy above 40 till the 2.0 update hit and people started leveling up. So or they removed NPCs or they toned their level down to make them harder to spot? Not sure.