I am not sure I agree as to any potential loss of value due to a well installed pad, depending on where your potential market for the resale of the gun might be:
If you are forced to only sell the gun in the UK, I would agree. That's unlikely, however. If your potential resell market is in the US, a well installed pad seems to be not only well accepted here, but actually desired in many cases, and the value would tend to be undiminished.
The key to valuation estimation (appraising), whether it be nice old guns, real estate, collector cars, or whatever, is market acceptance of the particular feature and RECENT prior market reaction to the same or similar feature. It's been my field for 42 years. Not to say I am a gun appraiser by trade, but the principles of valuation are the same whatever the field.
Therefore, an answer to whether a well installed recoil pad installation detracts or adds to value on a Greener, or on a Purdey, or on a Parker or a Crescent, could be found by researching a well maintained database of prior sales, and, within that, enough sales of a certain or similar model that would distinguish between sales with a pad and sales without a pad. I do not have such a well ordered database, alas, and my answer is only my opinion, as are those opinions above. I have managed to keep many sales data stored over the years, the basic problem with many of the data points is that they are not detailed enough to indicate whether a pad was on the gun at sale or not. As in any database storage example, the old computer addage applies, GIGA (garbage in/garbage out.)
Bottom Line: I tend to believe, based on simply observing many prior sales of nice old guns over a long period, that many potential buyers in the US like a well fitting and installed recoil pad on their shotguns, and that such a pad would likely not lower the value in the US market. My opinion only.
