It's not so much the peak power where the "p" heads shine, but their torque curve. A lot of guys who are running basic P-combo's tell about all the low end torque they have gained, and a few have 12 sec time slips.
A friend of mine slapped some untouched P's on his 5.0 with a Cobra intake and B303 and was trapping 13's ~ 106MPH with it. Prob could hit 12's with some suspension tweaks. Pretty damn good for "junkyard" cast heads.
My point wasn't to say they could make 300 rwHP, but more that to do so takes some combo tweaking and thought...as well as a little money.
However, there are guys putting down 270-280 rwHP with P heads without the exotic hardware. I beleive 25thmustang's dad is putting down 280 HP or so to the wheels with a stock cam and untouched P heads. Impressive number when you consider you can find these heads for $200 or out of a boneyard for even less. Slap a set of springs on them for $100 and you can put together a decent 300 flywheel HP combo with some good parts selection for fairly cheap. The P heads are less about peak HP and more about the torque they make. Fun street car combo that a lot of guys would be satisfied with.