All said and done, I have seen quite a few people shot. If the bullet hits the right spot, AND goes in deep enough, it shuts them off. A big fast bullet, that doesn't hit the magic spot works just as well as a slower, smaller bullet that doesn't hit the spot.
With modern ammo, most of the common defense calibers .38 and up, will, generally, go deep enough to reach "the spot" (whatever it may be).
You can shut off a biological creature several ways.
"Oh poop, he has a gun, I'm out of here" (only works on humans)
Get shot, it hurts, I'm done.
Get shot in the brain or spine, body shuts off, they are done.
Poke holes until enough hydraulic fluid leaks out, they quit working.
So, lets say we get a shot at the spine. A 9mm misses. Would that extra 1 MM in a .40 have hit the spine? Maybe. Would the extra 3 MM of a .45 have hit it? Maybe.
Can you shoot the 9mm dramatically faster than a .45? If you can shoot 3 9mm's for every 2 .45's do your odds of hitting the spine (aeorta, brain etc) go up? Again, maybe.
Will the three 9 mm holes cause more fluid to leak out faster than the two .45's?
I don't think there is an answer. No matter how hard we try, no matter how hard we seek the single answer, its not there. If there were truly a definitive answer, we would all be carrying the same thing because it has proven it is the best, under all conditions for all people.