See when you get into coaches at "all levels", that really leaves too much open. I could possibly be included there. But enough about that.
But you did mention Jack McDowell, Stanford alum. Not that you said you actually met him or talked to him, but that's not the point. McDowell is reportedly a bright guy. I can see trusting his opinion, assuming you ever heard it.
But really, should we trust McDowell?
I don't know if he believes in clutch. But I do know he doesn't believe in pitch counts, and thinks they're useless and bad for the game. This is in spite of the fact that his career probably ended prematurely because no one was monitoring his pitch counts.
So if players do believe in clutch, is that more or less relevant than them believing they need to leave the car at exactly 33 past the hour or need to eat chicken before every game? Is "clutch", whatever it is, another superstition?
As for believing the players, should I also believe them when they say corked bats help them hit home runs, despite the FACT that cooking a bat is about the worst thing you can do to one? (Although to be fair, sometimes it does enable a hitter to generate more bat speed. But this only works because the first bat was too heavy and the hitter should have simply gone for the perfectly legal maneuver of using a lighter bat.)