I practice the standard Moz drill until it's second nature, but when I try to do it super-fast, under stress, at ranges outside 10 yards I whiff that headshot about half the time.
I think the most effective thing you can do is practice a wide range of drills, and when the time comes you'll make the best of your training. No single plan of action fits all, or even most scenarios. The best you can hope for, imho, is that you have trained to the extent that firing and loading your pistol is as natural to you as moving and seeking cover.
One point of some interest. If you watch security camera videos of gunfights, as they often pop up on youtube, etc, you'll notice an interesting trend. Very often it seems that as soon as the victim goes for his gun, the aggressor, despite having the drop on the victim, tends to shag it. It seems that having a gun and brandishing it in the commission of a felony doesn't necessarily mean that the aggressor has the stones or the plan to actually shoot someone on a moments notice.
Not that I would count on that of course, just something I've noticed lately that I thought was odd.
My personal experience suggests that "in the moment" your training will guide you as best it can, there won't be much conscious decision making about it.