What you read on that site is completely accurate. It is known as a "PI headswap", and is good for about 20 RWHP. He will NEED a new intake to match the heads, which will add another $300 to the parts bill.
goon,
I see where you are coming from, but we are not talking about a CV vs a CVPI, but that FMC changed the design of the 4.6/5.4L cylinder heads in 1999. The "new" heads have teardrop shaped intake ports, and smaller combustion chambers, along with larger exhaust valves and "hotter" cams. Togather, these parts are good for 20 RWHP or so. The heads were named by FMC as "Improved Performance", but it does not roll of the tongue as well as "Performance Improved" so that's what people call them.
CrownVics did not get thses heads until 2001, all other 4.6L applications had them in '99.