PI is Performance improved. 99 and newer Mustangs, 2001 and newer Crown Vics, they others I don't know much about (trucks).
The pistons, heads, cams, lower intake plenum are different than the Non Performance Improved parts. PI heads on an NPI block/pistons yeilds a higher compression ratio. I think it ends up being 10.5:1. Okay for NA, not good for forced induction.
I already did the PI lower intake plenum on my 97 Cougar. The design of it seems better for higher end breathing, the common plenum is larger, the runners are longer.
I have the cams sitting waiting for me to get off my butt and have enough time to dedicate to doing them. I am keeping the NPI heads and only going with the PI cams.
With your 94, you can't just do the cams, you need 96+ NPI heads (which were made in 95, but nothing or next to nothing got them in 95) or go all out and do the PI head and cams. Since you have to pull the heads anyways, that might be the hot ticket.
There are a ton of posts on this here and other modular Ford sites.
97 Mark VIII LSC/Silver/Gray leather
97 Cougar 4.6 Sport/White Opalescent Tricoat Metallic /Gray leather down but not out?
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00 Grand Marquis LS/Vibrant White/Gray velour
63 Falcon 302/black and rust/shed isn't moving
82 F-150/Oxford White/dumb VV carb doesn't pass emissions