Whoever is promoting the SST lift as a leveling kit is incorrect. Some may call it a Level Lift as it uses spacers but is also a No Cut Kit (no differential drop) The RL 3.5" SST Kit is a lift kit (3.5" front and 2.0" rear lift.) ....Leveling kits are Front only, RL does make a 2" leveling kit w' UCA's.
As for why 5160's....no great reason but they were the Best available from Bilstein, great value for a Resi. So far back end of the truck has felt so stable on some back roads. 5100's In front are tuned for the added preload so they feel great in the front and the spacer just gives it the extra lift. RL also in my opinion comes with a better UCA.
If i could compare the feeling of stock shocks to my Bilstein setup, I feel the road more now but I'd say 75% of the is from the F Load Tires. I will also add that none of what I feel is harsh to me at least. Biggest difference that I feel is that on medium to big compressions, the Bilsteins recover faster. where the stock shocks resonate (bounce) back and forth a bit before leveling out, the Bilsteins compress then level out quickly making the ride to me more stable. An example of this would be when I pull in to a friends driveway, when my truck was stock the truck would rock back and forth 3-4 times (driveway has a big depression on right side) before leveling out, now it rocks to one side then recovers.