If you are willing to pay 5k+ for a hammergun that has been sleeved and has also had an extension done, I'm glad its your money and not mine. I can care less what the name is on the gun, that matters not. What I care about is the overrall quality of the piece. That Boss will sit at WLM for along time as will most of the sleeved guns a Vintage Doubles and other shops. The only reason I could fathom someone paying that kind of money for a gun in the described condition would be if that person had to have that particular gun in the safe to complete a collection, usually these types of people/collectors are looking for a very particular action, patent, style etc and MUST have the gun. Others who hunt and shoot their guns would be looking elsewhere to spend their hard earned 5K. A Purdey is a Purdey and sleeved Purdey is just a "SLEEVED purdey".
Dustin