Impossible to say, but probably not a whole lot in the scheme of things unless it was a rare example. Some enthusiasts want every piece of ephemera and are willing to pay for it, but a 20 year old common Rolex wouldn't attract that much premium if it had the original certificate, imo. It's very difficult to play 'what if' scenarios.
Here is an example of something I just had in a similar vein. I consigned a small (32mm) 18k gold Svend Andersen world time watch that I acquired in 1993 to auction in NYC that ended last month. We agreed on the estimated price of $3,000 - $5,000 for the watch, and a lot with an estimated price of <$5,000 carries no reserve price, i.e., whatever the hammer price is, it is sold. So if only one person bids $200 and no one else bids the watch sells for $200 + the House's premium. The watch had never been serviced and carried no box or any original dated papers, but was in almost mint condition, I hardly ever wore it. Just the watch, small by today's standards, but by a now well known independent watchmaker, his first serially produced watch. To everyone's amazement the watch sold for $19,000 + a 26% premium = $23,940! The CEO of the brand was so impressed he called the auction house to congratulate them on the sale. So a great payday for me for a neo-classical watch that at least two people wanted badly. Would it have made any difference if it had the original box that had long deteriorated into pleather dust, or some dated and signed document? Who knows, and it's just not something you can worry about. Whenever I sell a watch it is time to move on, I won't question whether I could have gotten more from a different venue, or selling it sooner or holding it for later. It's a moment in time, no more.
