Don't think I explained myself properly. I'm asking this because I've seen a few threads on car values/ebay adverts where most posters have said "Because it's modified it's worth less".
My car's only got a Stage 1 Kit, new wheels and arches. The big question for me is would my car be worth the most with the standard arches on? I.e. Would Group 2 Wides reduce the value? And, following on from that, would W&Ps?
I've still got the original wheels (the Rover 4.5 x 12" copies of Minilites), so if I really wanted to I could make it look like standard again.
In my opinion, i dont think thank changing the arches is going to affect the resale value of the car at all. Unless you've got a special edition which can fetch a lot of dollar for a totally standard car, its essentially negligible.
It all depends on who you think will be buying your car when you sell it on. If you don't mind selling it to someone who mods minis themselves or has done previously, then merely changing the arches for W&P over Specials is not going to bother them - they're probably thinking more about putting sportspacks or group 6's and 13x7's! The name is all elementary and doesn't really mean that much. W&P replicas won't add or detract from value. Real ones might be different.
Even if you only want to sell or advertise to mini purists, i still wouldn't see why such a small change to stay within the law would affect the value of the car at all.
If anything it would raise the value, because anybody who knows anything about the legality of cars etc is going to know you can't have any tread on show when looking vertically down from the arch.
So long as you dont need to cut any of the arch it could be argued you're not even really modding the car as everything can be put back to the way it was before.
The conclusion of this essay would be that i wouldn't worry too much about the value changing by significant amounts (and if the buyer wants to put it back to the original arches, keep them handy just in case)