OWD
You seem to be somewhat fixated on trying to prove the truth of a 'one size fits all' answer to the question.
The answers are somewhat more complicated because not all guns start out the same and re-stocking and re-barreling do different things to value. Re-stock a 1930s Purdey wrong and you will make it hard to sell. Re-barrel the same gun short and the same result. Have H&H 'Royal Brevis' re-barreled by the maker to 29" rather than the original 26" and watch it get easier to sell!
Re-stock a short gun to 15" with the right stock shape and to 15" - easier to sell.
An original, mint 30" barreled Purdey with original, unmarked 14 3/4" stock - very easy to sell, expensive. The same mint action, re-barreled and re-stocked by the maker to the same dimensions, the original gun will shade it as it will appeal to collectors, who would not be interested in the later. Plenty would be 'though.
In the real world, we rarely see mint, unused examples of guns which have been re-barelled and re-stocked. Just as we rarely see original guns in unused condition.
Im most cases we are comparing guns with some degree of wear to their original parts with guns with new, much less worn parts.
As many have said, too many variables to give a pat answer - not sure why you have such a bee in your bonnet about this one.
