Take this with a grain of salt. The body lift will not decrease your payload at all. It will also not affect your gas mileage, I drove my 3" body lifted pathfinder to the other side of the continent and back and still averaged 23mpg like I did when it was stock.
I haul stuff all the time with mine, I have a 3" suspension lift now as well and if you think that you can still rocket around corners like it was stock, well you're an idiot. It still tows great, although with the 33 inch tires it will downshift a lot more.
If anything my payload has increased due to the higher spring rate of the lift coil springs.