Manufacturers set the capacities based upon stress for all components. When you add aftermarket items you aren't affecting all of the components, just suspension. So the payload capabilities of the truck will not change. In order to change the payload capabilities you would have to make changes to all of the components the manufacturer looks at when they set the payload rating.
Two of the main items the manufacturer consider is brakes and tires. Most RVers fail to realize braking and tire life are greatly affected when towing over capacity.
For your safety you should have a tow vehicle with capacity beyond what you need instead of "at" or over maximum capacity.
That being said, I have towed for many years with a truck that was 3000lbs over capacity. I knew it was a risk and could come back to bite me, but I did it anyway. Now that I have a truck that has way more capacity than I need (26,000lbs vs my 15,000lb trailer). I will never tow above capacity again.