David
You are absolutely correct - you won't get your money back on resale. That is true for almost any modification. So the questions you should ask yourself is, "Will i get my money's worth out of the modification before I sell the car", if the answer is Yes, and you can afford it, go for it.
Mind you I had the same interest - we had adaptive on the order and last minute the financial controller said she didn't like the large wings on them, so we dropped them. Bad decisions. Last year I looked at getting adaptive sports seats but looking at the cost took the view I wouldn't get my monies worth out of them before I sold the car, so spent the cash on performance upgrades instead.
I kept an eye out for second hand seats but even then they are £2 -3 k to buy. So I took the view of either keep the standard seats or get bucket type sports seats which were much cheaper to buy and fit. Being a large well built sort I discovered that the buckets aren't really for any one wider than a pencil or over 5ft 8 in, especially not if you have to commute in your car. So I stuck with the standard seats and promised myself that the next car will have the adaptive sports seats.