Multiple thin leaf springs will always ride better than one thick leaf addition. For this reason, I like the design of the Calmini and Deaver packs, though I have no personal experience with either. However, I have had first hand experience with both other types of add-a-leafs, ie, thick and thin. I had a Jeep YJ with a single thick add-a-leaf on all four corners for a lift... talk about a teeth-jarring ride. However, I also built a custom set of leafs on my Samurai with Jeep YJ springs, using a couple extra stock leaves from another pack... rode like a dream.
Another consideration... multiple thin leaves also flex better than one thick leaf. That's one reason Alcan utilizes so many leaves in a pack.