I got this from another forum by the very knowledgeable blackfang.
"The PI intake is a considerable improvement over the nPI intake. With its increased port volume and improved runner design, this intake has been shown to give up to 18 RWHP increases to the nPI equipped Stang. "
"When swapping the PI intake on nPI heads, make sure you purchase nPI gaskets since these gaskets seal the water ports. Failure to seal the water ports can result in coolant leaking into the combustion chamber and subsequent engine failure, so be sure to use nPI gaskets. Although these gaskets effectively seal the water ports (a must), they do not completely seal the intake ports. You must use high temp silicone sealant (permatex) to seal these ports. AGAIN… only apply the permatex to the intake. Applying excessive amounts of sealant can cause it to ooze into the intake port and obstruct it resulting in hesitation and reduced power. "
The OEM Ford unit is better then the Dorman Replacement ones too.