PI heads are either "Performance Improved" or "Power Improved"
depending on who you talk to. They brought the non-mustang applications from 215HP to 237HP(ish). Some police cars (I think 04 and up) went up to around 250, but they had the marauder air box and MAF. The Crown Vics never got the Mustang spec cams, so never hit 260HP like the GT Mustangs.
DISCLAIMER- I drove Crown Vics for many years as a police officer, and have never personally owned a Ford 4.6L engine, although I worked on my former in-laws vehicles to include a '95 Lincoln, '03 Marquis and '03 Lincoln that had the 4.6. I have had a '92 Crown Vic, a '95, a '96, a '98, and a 2000 assigned as patrol vehicles in my career (which I pressed pause on as far as law-enforcement goes in 2008). Great engines, great vehicles. I would love to have one, but am by no means an expert in all that is Ford....see my sig