CJ,
I dunno how it's viewed by anyone or how it would affect value, but I did chamber sleeve a sxs .410 once. It's pretty straight forward would be nearly invisible and undetectable unless someone knew what to look for. Even then it would be tough to detect. If done well, I see no desirability issues unless you were dealing with a very pricey collectable. From a proof perspective, my understanding is yes, it does affect the validity of a British proof. Of course, in the US, valid proof is not required.
