I could go out and test with and without clipless pedals and come back with power data saying which system allowed me to get the most power out over say a 3 minute climb, however, I won't do it for the simple reason, I cba to waste a hill rep on such a futile exercise.
Additionally, the data still wouldn't really prove/disprove the "instant" or "step change" claims, since I am very much used to riding clipped in and have developed a pulling technique, someone who just changed over wouldn't do this.