I have always thought that the PI system applies to stock / upgraded cars alike.
In JonK’s quote he never mentions stock until that last paragraph.
My reading of it has always been as if the car in whatever form it is in is taken virtually around the test track and PI calculated.
All I know is the difference between upgraded builds for any given pi is much larger than it used to be.
Max I agree that there is no other official info so this is all we have to go by. I would love to know the devs thoughts on the current PI system and why it is the way it is.