You guys make this sound impossible. I drove a linked XJ for 3 years. Put around 15-20k on it. Many highway trips that were 6-7 hours round trip. The thing drove great. It rides beautiful. It had some excessive body roll on quick corners, but nothing bad enough that I ever put a sway bar on it. I have done 2 other XJ's with the same results. One finger on the wheel driving straight down the highway.
A leaf sprung XJ will never climb as good as a well linked XJ. Its just not going to happen. I have never had a set of leafs that were stiff enough to last, yet soft enough to flex worth a crap. After you wheel an xj with a balanced suspension. You would never want to go back to leafs again. At best with leafs the back end is doing maybe 30% of the work.
I am driving a XJ right now mainly just to run around town, get parts, ect. When I don't want to drive my truck. I want to link the rear of it for one, because I can, two for ride quality, and three its just smoother. The only reason I can think of that leafs are better is just simplicity...
More flex is "almost" always better. Even if a tire is hanging with no spring weight on it, The weight of the axle, tire, wheel, brakes, ect are not trying to pull the jeep into a hole. It is more stable then having a tire a foot in the air teetering back and forth. You might not think its a huge deal, but it makes a big difference. Its a huge difference in comp rigs when you are talking tires that weight 400lbs a piece. That tire with no spring weight on it, is still a lot of traction.
As far as the clayton kit goes. I have seen it in person. It looks good and is built well. I have not driven a rig with it, but I would guess the road manners should not be an issue. Just some added body roll. Now if you were to run stiffer rear springs, that would help the issue, but a sway bar would be the better way to go.