...it does seem like kinetic energy should factor into the 'stopping power' of a handgun round to some degree or other.
Kinetic energy is one factor that should be understood--momentum is another.
Kinetic energy is the potential of a projectile to do work/damage. The less distance it takes the deceleration of the target material to reduce a projectile's kinetic energy by a certain amount, the more force that is applied to the target material.
Momentum (power factor is a scaled version of momentum) relates to how hard it is to stop a projectile. The less time that it takes the target material to reduce a projectile's momentum by a certain amount, the more force that is applied to the target material.
So a moving object applies force when it hits an object. How much force is applied is determined by how much kinetic energy the moving object has and how much distance it takes to stop the moving object. The shorter the distance the more force is applied by a given amount of kinetic energy. The more kinetic energy stopped in a given distance, the more force is applied.
It is also true that how much force is applied is determined by how much momentum the moving object has and how fast the moving object is stopped. The faster the moving object is stopped the more force is applied. The more momentum stopped in a given amount of time, the more force is applied.
But kinetic energy and momentum are just the most basic aspects of terminal performance.
How rapidly a projectile stops and how much distance that takes is also dependent on other factors. Bullet profile, bullet size, bullet expansion and/or fragmentation, target material hardness, etc. can all affect the amount of force a bullet applies to the target material.
And the amount of force applied is pretty much irrelevant if it's applied to a part of the "target material" that will not cause incapacitation if damaged.
It's pretty easy to see that increasing all of the parameters at once will probably help terminal performance, but it gets much less clear when you try to prove how MUCH it will help. And when two parameters go up but another two go down, then it's really difficult to get a feel for what's happening in terms of effectiveness.
And finally, there's a problem with increasing all the parameters at once. You might get a gain in terminal performance but you trade away other things that are also important. We all understand that shot placement is important--therefore it follows that shootability in a self-defense weapon is also important. Since we understand that missing is inevitable and attackers don't always come by themselves, we all understand that having an extra shot or two of capacity can be useful at times. These, and other practical factors, are affected by caliber choice and therefore should be considered as part of the decision making process.
You can't ignore terminal performance differences, but it's just as misguided to pretend that caliber selection is only about terminal performance differences. Caliber choice affects other things that can't be ignored if the goal is to make an accurate and informed decision.
Finally, it should be obvious by now that what's optimal for one person may not be a good choice for someone else. I have guns that I would recommend as good self-defense weapons that my wife can not operate properly or shoot well. She has some self-defense firearms that work well for her that wouldn't be the best choice for someone with normal hand strength and recoil tolerance.
