The PI and NPI have the same PEAK flow. That is, they flow the same amount of air at .500 lift. Below .400 lift, the NPI's suck. The valves spend very little time at peak lift, most of their time is spent at low lift. Ask any head porter how important low lift air flow is and they will tell you that it is more important than a heads peak flow.
Yes, you can port NPI heads and make them flow very well. In fact, if you have a supercharged 96-98 GT with the stock shortblock and don't want the high compression ratio that the PI's will give you, porting the NPI's are the way to go. But it is cheaper to buy a set of PI heads than it is to get NPI's ported. If you don't have a blower, PI's are the superior choice.
PI heads aren't just for race motors. Every 99-04 GT has PI heads and they are very streetable. You just don't have a damn clue as to what you are talking about. Get your facts straight before you try to tango with me on engine theory. I'm one of the best on this board.