So, I can go ahead and shed some light on this from the perspective of a racer. I'm a big sprinter, and I've clocked myself in a sprint for both top speed and acceleration speed (my jump). Comparing my numbers with my Look Keos and normal platforms, I can tell you that overall speed isn't really effected by pedal preference, but my jump absolutly is. I can accelerate *much* faster with the Looks, simply because you can use more muscles for the upstroke. The platforms don't really effect my top speed at the end of the sprint however.
But yeah, say what you guys want about cyclists not actually pulling up when they wear clipless, my acceleration from between 15-35 mph is *rediculiously* improved when I use clipless, and I don't know what else could contribute to this.