I think we are putting to much into this. In a stressful situation do you want to take the time to try to figure out, by the way the target is standing, where the gizzard is?
If you do your (defensive) practice ALWAYS shooting center of mass, when you have to shoot, muscle memory is going to force you to shoot center of mass.
If my life is at stake, I don't think I want to take the time to figure out where each part of the innards are.
When I have new shooters shooting targets with scoring rings, I turn the target around so they can't see the rings. I tell them to shoot the center of the target.
Of course target shooting is different, so is the time required to get off the shot.
Army snipers are not taught to head shoot, they are taught to shoot center of mass. (LE snipers are different, but that's another subject).
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071