Not so much speculation, more sensible reasoning. IF the vehicle is totally level front to back, and you place significant weight, the end of the vehicle holding the weight will droop under the load, and sit lower than the other end. ALL vehicles when loaded with sufficient weight lower, and then come back up when the weight is removed. - So far, makes sense, as spring height reduces due to weight. In fact, the spring rate states how much weight it takes, to deflect/compress the spring 1". If you want to be more precise, and go a step further, the farther behind the rear axle the weight is located, the more the vehicle stance will be affected, so weight at the hitch (as far back as possible) will induce more rearward drop than weight located at the front of the bed, in front of the rear axle. (Weight between the wheels is more likely to evenly distribute between the two axles, the weight acts using the Archimedes principle, you're creating a lever, with the fulcrum being at the axle).
Given the leveling kits I've seen appear to be either 1" or 1.5", it's safe to assume there's at least a 1" rake front to back, possibly as much as 1.5", otherwise we'd be pushing the front above the rear, by installing a 1.5" leveling spacer in the front.
Now the factory springs vary in rate, depending upon FWD/AWD, Towing, and FX4 packages, however, I have it from a source, that the 4k Tow might just happen to be around 425 lbs. (Don't ask, I can't say at this point). That means from a unladen length, 425 lbs results in a 1" drop. In pairs, that's approximately a 1/2" drop with every 425 lbs added. Force vectoring springs work a bit different and may not be linear, so results on models so equipped may vary slightly, primarily your FWD models. If you're perfectly level, toss in 425 lbs into the bed, the rear will drop approximately 1/2 an inch, at 850 lbs 1", and at 1275 lbs, around 1.5" drop. (Slightly less if against the cab wall, slightly more if behind the axle).
The point is, by leveling, the back end WILL sit lower than the front, if you add enough weight to result in droop. The amount will be determined by the exact spring rate of the model purchased, and amount of weight, but for the sake of argument, 425 lbs would make the rear end 1/2 inch lower than the front.
Thus if you’re towing w/ maximum weight you’re looking at 1/2”drop, less if you’re towing less so visually you probably can’t notice. Less than 600 pds. In bed probably won’t notice since most of the weight will be pushed forward.
