That's why slope was created. The so-called scratch players of today are immune to slope because they have a 0 handicap. If they had the 4 they really ought to, slope would affect them and give them an extra couple strokes on a really nasty layout with 150+ slope.
Not that I believe slope is anything like a perfect model for the difference between scratch and bogey, let alone between scratch and almost scratch. Nor is it necessarily going to properly account for the effects of a really tough setup, even if you believe it does work well for the difference between "pretty easy" and "kinda hard".
I also think that a lot of the reason the top amateurs shoot a pair of 80s and go home in the majors has a lot more to do with their lack of ability mentally to handle the pressure, crowds, playing alongside the world's best, etc. than it does their skill. Some of those "local scratch hotshots" can match the scores of touring pros (at least on more normal courses) in a friendly game. I know one myself....if he maintained a handicap he'd probably be a plus handicap (and ahead of Tiger in driving distance) but he's beat several touring pros in friendly games, including a recent major champion.
If you sent him to Oakmont to play the course last Monday in US Open conditions with no one around, I don't know what he'd shoot, but I'd be willing to bet big money it'd be 10+ strokes better than he'd shoot if you tossed him on the first tee the previous Thursday!