Never heard Massad say anything about it, but I was trained by an instructor to shoot for the groin/hip area AFTER a few rounds center mass did not stop someone. I guess if you're unfortunate enough to deal with either someone on PCP or wearing a vest. The idea was that there are alot of major arteries in that area as well as bones that help quite a bit with mobility. Take that out or cut a few arteries and I guess they'll at least bleed out quicker.
Granted this is not something I would think would be likely, but it was a nice "well if all the above doesn't work" moment of training. I still train to hit center mass though.