NPI heads will outflow PI heads any day. Now, our stock NPI heads will not out flow PI heads, but 98 mustang NPI heads will. Of course you will have to do a lot of porting work on these heads, and so will you on the PI heads.
The reason the 98 heads are better is because in our heads, in the intake side, there is a water passage in there, you cannot grind away as much metal in the heads as you can on 98 heads(NPI of course). The reason I say 98 because they have a casting number, (don't remember what it is right now), on them that identifies what they are. I think there was some in 97 that had the same casting numbers.
Take it for what its worth.
If the PI heads where better, don't you think JL would be running them, and other guys for that matter.
I think a lot of people got in there heads that the PI heads outperform the NPI heads, in which they do not.
Hope this makes any sense.
May you fly low and fast
rest in peace JL