I'd take it to a different mechanic and ask them. Also make certain the person is also a authorized Shimano dealer or service center.

I just don't buy the thing about the RD. Otherwise why isn't everyone else griping about it here on BF and elsewhere.

The issue will almost have to be your free hub isn't doing it's job. However, you say you put another wheel in it. And that supposedly was with it's own free hub and still had the issue.

Perhaps it might be something else that is allowing the cassette to rub ever so slightly and that slows down the free hub. Maybe a spacer behind the cassette that shouldn't be there or something. But if it is, then that says even more reason why you should find another mechanic.

Or just learn to pedal going downhill. It keeps your legs warmed up and ready to pull at the bottom.