I noticed a modest improvement in handling in going from 3195@80 PSI tires to Nitto rated at 3750@80PSI. The 7500 lbs. for the rear pair provided for a payload of 4200 lbs. with my truck and that was more than enough.
I chose not to spend the $4000 on a set of 19.5 rims and tires as they were not needed, the ride without a load would have been much harsher, and the treads are limited to highway use types that would not be very good for use off the pavement. The added weight will result in more wear on the front end suspension and important to stick with the factory offset with whatever rims you choose.
With a 3700 lb. camper load the biggest improvement in handling came from adding an extra rear leaf to each side of the truck. Most of the motion was from the overloaded leaf springs and the greater range of travel made it difficult for the Rancho shocks to do their job as well. Part of the problem was that in a turn the downhill leaf springs were having to handle more than 50% of the load in the bed. Increasing the leaf springs load capacity from 4,000 lbs. to 5,000 lbs. made a big difference in handling.