I fly at 5200' as well and run a Saito 100 and its just awesome! I couldn't see putting anything else in it at this altitude. Unlimited vert and pulls out of hover with much authority. Saito 100 gets my vote..
Running 20/20 and propped with a 15x4W.
Also, I would keep it powered with a 4 stroke. My .02
Good luck!