Considering switching to 9mm from 40S&W

Background:

A couple of months ago, I was out shooting with a friend in the Middle of Eff-ing Nowhere. I had some new guns I hadn't had a chance to shoot yet, and my usual 40S&W Springfield XDm.

My friend also had his 40S&W XDm, and he decided to bring out his old Taurus PT99 9mm on the "I haven't shot this in ages" premise.

This was a regular shooting place, and the ground was littered with 12ga shells. One of the fun little games was to toss a handful of these out randomly and see what you could hit them so they'd go flying from 10-15 feet away. Its not a bad test -- if you can hit something that small (or near enough to kick it up) from that distance, doing the "Mozambique Doubletap" (2 to the chest, 1 to the head) to a much larger target at a greater distance should be similar.

With my XDm, which I love, my longest streak of continuous hits was 3.

With my friend's 9mm Taurus (a gun I'm quick familiar with, I have to PT101, the 40S&W version), I hit 7 in a row, more than once.

My friend commented on this.

Which got me thinking...

I had a friend switch from 45 to 9mm because of his arthritis, and watched his accuracy skyrocket. I have another friend who only shoots 45 and 9mm, because he thinks 40S&W is too "tippy" (reacquiring the target is slow because of muzzle climb).

I bought my first gun (the aforementioned Taurus PT101) in 40S&W in the early 90s partially because it was the hot cartridge (luckily I avoided the 10mm!) and it was, I think, superior to the 9mm at the time. 
 
Now, in my honest opinion, with modern powders and super-exaggerated hollowpoint bullet technology (I think Golden Sabers and Black Talons had just come out when I bought my 40), a 9mm is just a deadly as a 40S&W or a 45. As Say Uncle and several other blogs have stated recently (http://www.saysuncle.com/2009/11/19/the-best-way-to-double-the-effectiveness-of-any-bullet-is-a-follow-up-shot/) -- its far more about shot placement ("try aiming") and shot followup ("the best way to double the effectiveness of any bullet is a follow-up shot") than caliber.

From a 30 foot distance, my 40S&W grouping isn't bad -- its all in the inner kill zone:

But the actual grouping of shots together...not so much.

Soon as I get a chance, I'm gonna hit the range with my 40S&W XDm and a borrowed 9mm XDm. If the shot placement with 9mm is exceptionally better (I think it will be), I'll heavily consider switching to 9mm, as I can just sell the 40S&W XDm and get a 9mm XDm instead. Then I'll need to sell my 40S&W HiPoint Carbine, and the PT101...