The ability to change a car's set up should affect PP. Whether or not the driver is smart enough to use the enhanced parts should be irrelevant. When I add aero to a car, it should jump the PP some number. That part may be beneficial or detrimental, but the fact remains, I can control the characteristics of the car and thus my PP should be higher.
I think someone suggested a few pages ago (feldynn maybe?) that some parts should affect PP, but not necessarily very much.
I'd be okay with adding a single-disc clutch upgrade putting PP up by 1. Racing twin-disc PP +2.
And yes, I do now see why adding a fully adjustable Suspension should at least bump up your PP by a couple points, because some testing I did early on in GT5's life proved that simply adding it lowers your car by default. Sometimes a LOT. Even if you have the ride height set at 0/0, it's already lower.
I took pictures from the exact same vantage point and it was obviuos by zooming in on teh wheel wells that this was the case.
But after adjusting it, I would not want to see PP increase more.. Just fixing the part to the car and having it be like +4 PP or something like that, would be okay w/ me.
However, this presents a problem for the addition of teh adjustable LSD because it does not automatically change the car's LSD when it is added. It simply oopens up the adjustment of it. Some cars do not have an LSD until you add one, so in those cases pershaps the PP should rise by a couople points. In other cases, I would not want it to.