Aren't there stories of lucid dreamers who were actually able to show a measurable improvement in a given skill after practicing it in a dream? I seem to recall reading about that somewhere. If true, those stories would be at least weak evidence supporting that idea.

On the other hand, this should mean that humans raised in cultural and social vacuums ought to be disproportionately talented at everything, and I don't recall hearing of anything about that one way or the other, but then I can't imagine a way to actually do that experiment humanely.