I think professional restoration can sometimes increase the value of items. It very much depends on the item, the nature of the restoration and who is doing it. To use your firearm example, the average gunsmith restoration may not help the value, but send it to someone with the caliber of say Turnbull Restorations and the value will definately not be hurt and probably increased. In my family, great great grandfathers Winchester is very rare. My brother had the rifle restored by Turnbull and currently it's on loan to the NRA firearms museum. On the Antiques Roadshow I have sometimes heard them tell people with who have an item with a condition issues that the value of the item will be increased if they have the issue corrected by a professional conservator.
I view my Buck Knife collection as being an "Art Collection". That viewpoint influences not only the type of knife I buy but also the condition. I want my knives to look as good as possible, ie "displayable". If you search "Buck Knife" on eBay you will get over 8,000 hits, Buck knives from the mid 70's and newer are not rare so there is no reason to buy a knife with condition issues. I'll bet 95% of my collection for modern Bucks is NIB or As New. For earlier Bucks, scarcity and value has become an issue. Assuming you could find it, any very early Buck knife in unused condition will be too expensive for me. Then I'll take the more affordable lesser condition knife with the hopes of the Spa treatment improving it's displayablilty, and displayability increases its value to me. Buck's Spa treatment is certainly affordable and it's also "factory", if that means anything. Is there someone else out there who can do just as good or better job than the factory? I've never seen any discussion on that and if there is I would love to know who.