In my 150,000 rounds on trigger, either rifle or pistol, I tend to use a simple black sillhouette and have memorized as best I can where high COM is. When I am instructing classes here, I tend to teach students to aim high COM, and skip the headshot except for certain drills. Yes, this may be a fallacy, but I have yet to see any distinct proof that a headshot is likely to be hit under stress in a bad situation, compared to aiming for high center of mass and putting as many shots as possible on target. We do run drills to do two to the torso and one to the head, and I'll have them do slow aimed shots, but on moving targets or targets that turn to face and then turn away, I find that the majority of shooters, even good ones, have trouble making the head shot due to smaller area vs. rushed response.
In my experience, the times i have been in "that" situation, I had quite a bit going through my mind, the least of which was evaluating the target for opportunities to make perfect shots. I merely aimed by instinct and concentrated on not jerking the trigger, while trying to remember fundamentals of getting to cover/shooting from cover.