I have the KORE HD Leveling kit on my truck. It was one of the first things I added. I am MORE than satisfied with the kit. The drive is MUCH improved. Makes it a whole different truck. The springs ARE certainly sturdier than the stock ones. My kit came with the Bilstein 5100's, I think the new kits have the Fox shocks? I am not sure about that. I also added the Bilsteins to the rear. The kit is around $600, can't remember, but I certainly believe it is worth the investment. There are also kits made by Thuren, Carli, Lorentz, all of them are good kits. Before the KORE installation I had some body roll and a little pogo in driving mountain turns, that was due entirely to the springs, they are really not up to par for our trucks. You will not overcome those type of issues with just adding spacers and shocks.
As far as the aerodynamics and fuel economy, my fuel mileage did not change when I added the KORE. The truck is heavy and the winds do not seem to phase it here in NM, and we get some terrific winds here. I also did not see a fuel mileage change when I added the BFG tires, but they are only an inch wider that the stock tires and that is a pretty insignificant change. They added a couple inches overall to the truck height, and that also did not make any difference in driving in the wind.
Hope that info helps you some.
2006 Dodge 3500 MC DRW 4x4 6-spd 3.73 LS rollin' on 19.5's. Not stock by any means . .