"Comparing one gun out-of-proof with an identical gun that is still in proof, the out-of-proof gun will be valued less in the market. Correct?"

maybe...depends if the buyer perceives the alterations to the out of proof barrels improve the desirability of the gun.

for example, some english guns have had the chambers lengthened. some buyers perceive that as an improvement.

some english guns have had the barrels reamed to remove pitting, in order to improve appearances. some buyers perceive that removing pitting is an improvement.

keep it simple and keep it safe...