I think the biggest factor here is green speed. Most scratch amateurs have never played a round with the green speeds at "tour event" level. If the scratch amateur was allowed to consistently play with those green speeds and get used to them, I think the gap would close. But if we are talking about a normal match at a tour level green speed course, then it may be close to 15 strokes difference. Now if we're talking about playing a course with greens that run about a 9 on the stimp, I'd give the edge to the scratch as far as it being less than a 15 stroke difference. Pros struggle pretty good when they suddenly switch to slower green speeds if they are not given time to adjust
This used to be the case, but for the last 10 years or so Tour green speeds arenít rare in the club golf realm. Iím excluding extreme set ups at Augusta and US Open, but run of the mill tour events arenít faster than you see in a state am or open in most cases. It isnít rare to see TPC Scottsdale at Tour speeds, same for Pebble, Riviera, John Deere, River Highlands, etc. Unless the weather gets out of hand while the tour is in town and itís windy, dry and sunny green speeds at the average tour stop arenít any different than ďpeak seasonĒ any longer. They arenít taken to the brink of death except for US Opens, Players and maybe one or two others any longer.