"Dan, my ref to a softer top layer of foam, according to my upholsterer, is mostly to help in making better corners, not seating comfort. "
Actually, the firmer the foam under the seat cover, the better corners it will make. Now I'm referring to seat covers that are sewn with sew foam, not necessarily to seat covers that are just fabric alone. In that case you'd need a slightly softer surface directly under the seat cover.
No one lives forever, the trick is creating something that will.