As a dressage rider, I am not a fan of foam panels. No, they cannot be stuffed to fit a specific horse, all you can do is pad them up. For jumping, they are adequate, but not at all acceptable for a dressage horse.
I know that somehow sounds judgmental, but a dressage saddle has to be changed as the horse changes his shape from proper riding; at least yearly, and sometimes every 6 months! A jump saddle doesn't the long points at the front of the tree, and the rider doesn't sit in the seat the same way a dressage rider does.
The foam panels will last a good 20 years IF the panels are cleaned, not over oiled (the oil will break down the foam) and not kept for long periods in extreme heat (like stored in a horse trailer).
There are some really wonderful hunt seat saddles that are wool flocked (Keiffer, Sommer, Barnsby, Stubben) and they need to visit the saddler every few years to keep them in good balance. If you tend to ride with more weight in one seat bone or the other (and most of us do) it will alter the side to side balance of the saddle.