We have 5200# axles as well with a wheel rating of 70psi which was stamped on the inside of rim. We used an e-rated 15" ST tire and filled to about 75psi with no adverse affects as it pertained to wheel failure. I imagine it would work as well for your scenario although no need to go all the way to 80psi as it would be a bouncy ride for your rig. ST tires perform better when aired up close to or at max psi stated on sidewall, filling GY e-rated tire to mid seventies will give you the added safety margin and would be less tasking on your current set of wheels.
