I think that the difference is quantifiable and real for any golfer. The width difference is more than the width of the pinky finger for me the difference between single overlap and 10 finger is in excess of 1.5 inches.
Possibly the interference that you mention is that the trail hand is actually slowing the club at impact in light of Peter's statement: The club is moving faster than the trail hand. If the trail hand was powering the club the club would be moving slower! The club is effectively a flail in the tour pro's swing.
Logically the further down the shaft the more the slowing.