Slushee...you ask a very good question and hopefully I can help out a little. Remember you get what you pay for and this is free advice!
The answer depends on 3 factors - the origin of the gun, the provenence of the gun, and the quality of the refurbishment. The latter point is obvious. The work should be done with original techniques (ie rust blueing if the original was rust blued, oil or varnish as appropriate, etc.) and to an equel or better standard of workmanship) Bubba anything, even a Norinco, and its value will go down.
The origin part is a little more subtle. High grade English guns were intended to be maintained, including re-blueing and re-finishing. This is why the wood was always left a little proud of the metal - to allow for subsequent light sanding and finishing. It is also why finishes were used that could be easily worked on. In other words, refinishing an English (and by extension European) gun should not detract from its value provided the work is done to a high standard. Careful polishing to preserve engraving, screw holes, edges etc. Blueing performed with the original techniques, checkering re-cut in the original style (flat topped, etc), and original type of wood finish utilised. "Strip, clean, oil and black" in the trade speak. Notice, no buffing wheels, cold blues from Cambodian Tire, epoxy spray cans of glossy varnish, etc.
The other exception to this general principle is providenc. If I had Theodore Rosevelt's Holland and Holland double rifle, I think that I would leave his rusty fingerprints etc intact.
American guns in general, and Winchesters in particular, are the complete opposite. People will pay more for a rusty gun (patina is the collectors term I believe) with a tiny area of original finish remaining than a professionally refinished example that is better than new. I really can't understand it, but without a doubt, this is the case. Perhaps it is because their sense of history is so young compared to Europe, that they subconsiously cling to any shred of it, who knows.
I have a great many collectable guns, some American, most English. I leave the American ones alone, save for cleaning and maintaining them. I build my collection by buying fine English or European ones that are in need of refurbishment (including re-stocking and parts replacement) at a good price and enjoy the hobby and satisfaction of doing the work.
Re: your Martini. They are sought after by collectors primarily because of their reputation for accuracy. I have 2, one, a Greener, that shoots five shot groups into 1/2" at 100 yards. Their value is not greatly tied to their condition but to the value of their actions. You won't increase the value greatly by refurbishing it (certainly not as much as the cost to refurbish it) but you will increase the pleasure of ownership. It sounds like it might make sense for you to do the many hours of hand polishing and draw filing and then send it out for a final polish and reblue by a professional. Drop me an email if I can be of any more help.
