I'm not entirely sure they hurt residuals. Actually, I don't know if they do, but I can't think of a reason they would(?). Granted, it's unlikely they'd add to the re-sale value quite the same as they do when you add them to a new car but if I were to buy a second hand GT3, I'd look at the state of the seats (worn, scuffed, etc.) rather than the 'type' of seat.
Personally, I think Porsche should offer only the sports seats then only one type of bucket seats for the GT3 and save themselves - and buyers in turn - the costs of offering 4-5 types. My thinking is that the standard seats are (or should be) adequate for road use and the Porsche buckets can be for those who track their cars on occasion.
If you're seriously tracking the car, get it with standard seats to save the cash and spend the difference on a non-Porsche race seat. They should be just as good (I haven't seen the Porsche seats in person, but have seen many race buckets in my time) and come in a range of prices to suit your wallet. And even your helmet if you want to use Hans.
Anyway, thanks for the info :) If they can be adjusted by that bunch of holes you mentioned then they should be just what I'm after :) I'm very much of the long-arms-short-legs persuasion so more often than not, I sit closer to the pedals but further from the steering wheel than most.