I can only adjust at the back end of the torsion bars, through the adjusting bolts. I had to read up on spring rate just now, but as I understand it, spring rates are constant for a certain spring. it's a measure of how stiff a spring is. as long as the spring doesn't physically change, neither does the spring rate. that's why you can find spring rates of a certain spring when you buy it.
but back to trucks - here's the situation when you "pre-load" (my example is a T-100 2WD). The truck is on stands, and the wheels and arms are resting against the bumpstop. The torsion bar is completely "un-loaded" i.e. no twist. so the bumpstop is absorbing the weight of the wheel/arm.
now imagine you have someone hold up your wheel, making it "weightless". you can now remove your bumpstop, and let's assume that without the bumpstop, you've got another couple of inches of downward travel.
now, let's say you tighten your adjusting bolt. tightening the bolt would just move the wheel downwards. you tell your buddy to move the wheel down when he feels the wheel pushing down on his hands ... in other words, don't be a bumpstop. when the bolt is tightened, the change in the angle of the arms would roughly match the change in angle of the torque arm (the whatzit that connects the torsion bars and adjusting bolt). you're changing the initial position of your torsion bar, without any load on it. note that because there's nothing stopping the arm from going down, the bolt will "tighten" very easily - there's nothing pushing the bolt and the nut apart as your friend is keeping the wheel weightless.
now, you do the same to the other side, and put your truck on the ground. it'll be higher, yes. that's because the starting position of the torque arm, torsion bar, arms, and the wheel has changed - by "tightening" the bolt. and now tightening the bolt will be hard - because the the corner weight of the truck is applied to the adjusting bolt/nut. in this situation, no more force is transmitted than before you lifted the front end. the corner of the truck weighs the same. that weight is transmitted from the wheel to the arms, to the torsion bar, to the torque arm, to the adjusting bolt, and then to the frame.
why the preload, then? it's because Toyota designed the "unloaded" starting position (through alignment of the bumpstop, arms, t-bar splines) for the torsion bar for stock height, not for us people who want a higher front end. If the bumpstop position for the wheel was lower, the torque arm starting position (unloaded position) would be at at a position where the wheel would be lower (and the truck would be higher), and you'd be higher after you put the truck on the ground.
the main point i'm trying to get across is that the unloaded position is just the starting point for where your torsion bar starts twisting. because of the bumpstop and the limit on how far our arms can drop, we can't set the starting point for our torsion bars to wherever we want. preloading moves that start point so your truck can be higher, but we can't see the suspension in that starting position because of the bumpstop. but as soon as the truck is on the ground, the force that the bumpstop was exerting on the torsion bar is completely replaced by the corner weight of the truck. that weight doesn't change regardless of whether you pre-loaded your torsion bar or not, so ****all else being equal**** your ride quality shouldn't change.