If its sitting on the bumpstops, the spring rate is infinite, because the springs are completely out of the equation. But otherwise, changing the height does ABSOLUTELY NOTHING to change the spring rate. The torsion bar adjusters control only the angle of the lower control arm, and do nothing to the bar itself.
As far as lowering a car using the torsion bar adjusters, there's no problem with this at all, especially if you're only looking for about 1". As others have already mentioned, it will reduce your suspension travel. But, as long as you don't lower the car drastically, this shouldn't be a big problem. Stiffer torsion bars would also help keep the car from bottoming out.
You will need a new alignment. As you lower the car, the suspension geometry changes. If its only a minor drop, then the car should have no issues being adjusted back to spec. But again, if you lower the car significantly, say more than 1", you may not be able to get the alignment back in order without offset bushings.
Drop spindles are a quick way to lower the car, I have a set on my Challenger. But, also know that drop spindles change the geometry of the front end also. Not alignment type stuff, but they will raise the roll center of the car, and increase bump steer slightly. If you can lower the car with just the torsion bars, this will give you a better overall geometry than drop spindles will. But, you may need stiffer bars and offset bushings, so there's a trade off.
