I don't know about gen 3, but my gen 2 squats 2.5" with 540 over the rear axle. I'd expect about the same with 480 against the tailgate and I'd expect the spring stiffness to be roughly the same, gen to gen, since it's tuned for the same purpose.You would need to keep 480 pounds of sand in the very end of the bed to move the rear suspension down 1.5" which would also kill 1.5" of travel in your suspension.
Yes, weight reduces up travel. But spacers reduce down travel. Pick your poison. You need both if you use your Raptor for it's intended purpose. If it's all about a look, follow your bliss.