x2. No way you can increase the payload or GVWR aftermarket.
HD payload includes a heavy duty axle and wheels that would be very expensive to do aftermarket. They increase the rear GAWR, and likely it's the rear GAWR that limits how much you can carry anyway (not so much the GVWR or Payload).
However you can improve the regular F150's suspension, which a lot of us have found to be just too soft even for our lower than HD payloads. And you can do that by looking at what the HD Payload does to its suspension: heavier springs, HD shocks and LT-metric tires.
There's lots of ways to stiffen the springs, I chose Roadmaster Active Suspension. For shocks, I had my Ford dealer order and install the HD shocks. And for the tires I got Michelin LTX MS/2 LT245/75R17E
These 3 changes have really improved the ride, especially when fully loaded, but I still only max load up to the rear GAWR of 4050#. And am usually several hundred pounds under the payload and GVWR then.
2011 FX2 SCab EcoB MaxTow 3.73diff ~2050#Payload LT245/75R17E RAS
New:2014 Timber Ridge 240RKS ~7500#Loaded ~1100#TongueW 10.0mpgUS
Old:2010 Trailmanor Elkmont24' ~4000#Loaded ~500#TongueW 12.5mpgUS