If you also put in a full race suspension and drive train as well as race wing and spoiler then it should be close and if not then the PI should not say that it is. That is the issue.
I don't think you understand how the PI works at all.
Let's say we have two cars here, both theoretically tuned to A600.
Car 1 is a Lotus Exige with slicks, weight reduction, everything that can improve its handling, and then about 325hp to make it up the rest of the way to A600.
Car 2 is a Dodge Ram SRT10 with the 540 Hemi swap, supercharged, all power mods cranking out 1072hp. Then add all the handling mods to it, short of the weight reduction until you get it to A600.
Both cars are A600. They should run identical laps times?
In 99.99% of tracks, no, they won't. Unless its an abnormally fast circuit with very, very little curves, the Lotus is gonna kill the Ram. You'll notice cars have a Speed, Handling, Acceleration, Launch, and Braking attribute. Somehow the combination of these is what determines the PI. If you had these two example cars, you'd see the Ram will have a much higher speed, acceleration, (and possibly launch...though actually launching a 1000hp truck vs. a 325hp MR car, the MR car is going to be much easier and quicker without any traction control) and the Lotus' handling and braking will be much higher than the Ram's.
Now compare those attributes of a C6 Z06 modified to R3 (R2?) compared to the real C6.R.
Team Corvette does not go to their local Chevy dealership and buy a new Z06 and turn it into their race car.