PI stands for power improved. Ford revised the 4.6L 2V engines in 1999. The 99-04 2V GT's have PI engines (or power improved engines). Basically, Ford improved the intake design, cam shafts, and cylinder heads to produce more power. This is why 99-04 GT's are rated @ 260HP while 96-98 GT's are rated @ 215-225HP.
You have an intake, cams, and heads from a 99-04 GT, giving you about +50HP or so. Its still a SOHC 2V.