I definitely agree with your position on unsprung mass. The difference between the Jamboree on original axles and the Naya Daur on FJ40 axles isn't very obvious on the road but off road, the original axles definitely allow for a better ride. As I read somewhere a few years back, compression damping controls the movement of unsprung mass, and rebound damping controls how the vehicle's body reacts to that movement. This is why I like RS9000s despite their short life; I can dial in just enough compression damping without really upsetting rebound damping. The progressive nature of leaf springs acts as a built in shock absorber so you don't need much compression damping, but it also releases that energy very quickly, so rebound damping needs to be stiff. The RS9000s manage this job beautifully.