I don't think it's such a simple matter as whether lower delivers more power to the pedals or "lower is faster" - casting variables aside then yes, there's truth to it.
Look at track racing or time trialists - a low, aero position for a short race over manicured conditions is going to produce a better result because we've stripped away as many variables as possible.
Over the course of longer events, fatigue starts to play the greater role in power delivery. As events get longer or surfaces more varied (stage racing, triathlon, classics, etc.), you'll see the drop go from 5-6 inches to 2-3 inches or less. You'll see pro teams use endurance bikes for spring classics because they can sustain a higher power output for longer in a more comfortable position.