There's no way to know what portion of the clues the human contestants knew the correct responses to. I've watched Ken and Brad play extensively though, so I very strongly suspect that they easily knew at least as many as Watson.

I would like some stats about that. As I said, Watson could have correctly answered 84% of the questions, nobody could answer 6% of the questions. That leaves a very thin margin for the humans, if past games are any indication one that is probably not large enough.