Tuning parts are mostly made of lighter materials so decreasing (at least) some weight would definitely help performing the care even better.
I mean what's the point of having 3 stages of weight reduction when you don't see it back at your car? for example in FM3 or even in NFS Shift, reducing weight by taking out the fancy interior and installing a race cockpit with just one seat makes perfect sense and is also visible after the upgrade has been bought, I don't see that in GT5...
After buying all 3 weight reductions there is no visible change with the car.
But I guess PD finds it more important to add 1000+ cars to the game then creating just half that much, but then do it properly... What a shame !!