I've never sold a leather piece with spring-down cushion cores in it, and I've sold thousands! Generally that is a cushion that is put into fabric upholstery as a premium core, they dent to not have enough 'bounce-back' in leather casings. What is sold most frequently in leather as an upgraded core is called an Ultra Down core (in H&M lingo). This is a foam based center with a layer of down and feathers on the top 15% or so of the cushion. I would imaging that was what you were sitting on when you tried the Sundance. Ultra downs will be more plush, but not spring back as fully as the standard Qualux foam core so they will give the piece a more relaxed look over time with some puddling of the leather as it wears in.