If deadly force isn't justified, it's not justified at all. If it is, the quickest and surest way to stop someone is to hit them in a place where they'll be dead in short order or immediately.
Sounds ugly to some people but that's what the army does, that's what the police does, and that's the most effective option available to the private citizen if the attacker doesn't stop at the sight of a gun.
Trying to pass off deadly force as "not so deadly really" isn't going to satisfy people who don't want to see guns in private hands.
At any rate, we digressed from the ongoing caliber war!
Reports on real world shooting indicate that there is tremendous variation in how different humans react under different circumstances when struck by handgun projectiles. There is so much uncertainty that there is no way to predict "ecactly" hat will happen.
I agree - and trying to do turn this chaos into sold statistical data would be a nightmare. In the meantime, we have ballistic testing which is based on quite a rudimentary approximation (ballistic gel). Personally, I don't think the methodology is accurate enough to judge real world performance of two rounds or two calibers which are close enough. It's a sufficiently good method to discount really bad rounds, that's all.
Between the rounds and calibers which perform well, in the end it's a matter of personal preference, and nobody (yet! who knows what they'll be able to model on a computer in another decade or two) is able to actually
prove that their choice is optimal.
For myself, if it ever comes to it, and I don't think it will, I'll be relying on the .45, but I got no beef about people picking 9mm or anything else they like. Hell, two world wars and countless small ones were fought using the 9mm in SMGs and handguns and it worked reasonably well - it's only going to be better with modern ammo. The .45 traces it's lineage to cap and ball days and the US civil war and there were people using Thompson SMGs in the war in 1991 in my country , and again, it's only going to be better with modern ammo.
I don't see the point of trying to cut it so finely to say which clearly adequate and proven handgun round is optimal, unless you're trying to sell something.