Does the value truly go down with a mod that is completely reversable? I know guys locally that own auto body shops and use their cars for advertisement at the local shows and such. It's reversable and they have the OEM parts available so would temp mods really permanently devalue your ride in the long term?
Any NSX can be restored, I think it's a questions of whether it's financially worth it.
If a car is modified but all the oem parts are available with the car, then a restoration would be simpler and cost less.
The car's value could the same as a stock unit or higher if the buyer views the modifications as desirable.
However if you've got to source oem parts or repaint to oem specs, the cost will be significantly higher.
That car's value could be expected to be lower.
Some NSX's today are not worth restoring from a financial point of view.
I don't think a unit with a widebody kit, non oem paint job, and no original parts stored away would be worth restoring given today's market prices.
From my limited experience, even with the original parts available, it looks like at least half the cost of modifications are lost immediately.
So if an owner spends say $10 K on bolt on body parts, paint etc. I think $5K is gone before the paint dries.
This of course ignores what an owner feels they get out of driving a modified version.
It appears that to many owners, particularly younger ones, the residual value of the mods and the effect on the car's value seem to be secondary to the desire to customize.
Valuation is a tricky subject but I think the less an NSX is modified, the more potential buyers there are.
Modifications, being a matter of individual taste, may eliminate some potential buyers who don't want those particular mods.
Engine modifications seem to be a double edged sword.
It seems most, if not all, potential buyers like the idea of more power but that is offset to some degree by buyer concerns over reliability and engine life
I've noticed two heavily modified NSX's on eBay not sell auction after auction.
The price shown likely reflects the cost of the modifications and while they may really appeal to the younger ex-Civic buyer, those buyers don't have the money to buy either car.
So I'm not sure my answer to your question helps but my conclusion is that anything done to an NSX ( or any car) that reduces the number of potential owners, also potentially lowers it's value.