From what I've read they are an ride better. I missed out on the euro wildtrak leafs off ebay since they stopped shipping to the US. If I make a move on rear suspension looks like ICON at around $700 is the most affordable option.
Multi-leaf leaf springs by design are progressive because of the varying length leafs. Single leaf leaf springs can also be progressive if the thickness of that leaf tampers toward the ends.
The Tremor springs are also dual rate because the shortest leaf in the spring pack does not contact the next leak in the pack. The suspension has to compress a ways before that leaf contacts the rest of the leafs in the pack and stiffens the pack.