The Art of Natural Dressage

Hi everyone,
I haven't been on this forum for a while and it is a question that brought me back after playing around with all kinds of tack all summer trying to find the kindest solution. A friend told me about Specialized (a brandname) saddles, a kind of saddle that can be adjusted with various types of shims to fit different horses. I am looking at these especially in comparison to Orthoflex and other Flex-Tree saddles and also treeless saddles. Does anyone have experience with this type of saddles? I'm especially interested in what material the tree and the shims are made of and how difficult it is to fit it to a horses back. Any saddle engineers out there who have comments?
Birgit