Basicblur, I think it might depend on what you interpret the size of the target in the illustration to be. The way I have always understood a sight picture and 6 o'clock sights is how I illustrated it. I have always held my sights level at the very center of the target with the understanding that the bullet would impact just over where you were holding the sights. I suppose a better way to say it would be to put an x right in the center of the targets in my illustration and that is where the impact should be.
I suppose I could be misunderstanding what each term means, but with my XDm, you have to completely cover the target to hit the center of the target. As in put the white dot of the front sight on the bullseye and you will be on target. Even at close ranges, say 7 or 8 yards, if you aim as if they are 6 o'clock sights like in my illustration, you will be at the bottom of the target, 8-10 inches low.
It could just be how I shoot, but I know for a fact that many other people shoot the same way, at least the people I have talked to about it with and shoot with.