Agreed Nate, just parsing the info for theoretical purposes. The capacity argument is a good one and very compelling. What isn't always pointed out is that it isn't a given one will be able to effectively employ the additional rounds - sometimes (unfortunately) the best one can do is land one good shot.
Now, my controversial opinion: I remain unconvinced that the impact energy (oh no, the dreaded "knockdown power") is never of any significance. Maybe it never is, but when I look at the difference on steel or pins, I still can't ignore it. That said, do I think the 9mm is effective? You betcha'.