If I was defending myself, I'd only go for headshots. It's all a matter of numbers. Get enough bullets in one direction & of them is bound to hit.
I've never been in an actual gunfight and hope I never end up with one, but I have been in situations where you have to engage many targets on the clock. Your hands naturally point at upper chest level. By the time my gun is pointed straight ahead, I'm already squeezing the trigger. With an unholstered gun at low-ready, I can put a round into a chest-sized target out to 10m in half a second. Even if it involved no aiming (lol), getting it up a little higher for a headshot takes extra time. Assuming you were an operating operator, operational in operations, the grandaddy of all Tier-1 guys and didn't need to aim to get a headshot at a moving target, you'd have 2 projectiles in your chest before you could get your gun up to my head level, and I'm sure there are at least 5000 guys and gals in Ontario that would make me look like a total amateur. Now, if you're sure you're engaging the last threat or know the target is wearing body armour capable of defeating your cartridges at the engagement distance, you can walk your rounds up to the head level, but it involves starting out at center-mass and just pumping out a few extra shots. As for your numbers theory... How are you gonna send "enough" bullets the other way if the other guy pumps you full of lead because he went center-mass?