The No Stripe had drastically different aerodynamics in GT4. The 300 MPH Club had known it for a while, but no one had ever bothered to explain it to anyone else. It was actually a pretty big deal when it was found out, because the GT was used in one week of the Tuning Car Competition and I pointed out the glitch with the car to them somewhat late in the event, and it had to be restarted from the beginning with the No Stripe disqualified from it.
And since all of the cars in GT4 were carried over to GT5 unchanged, I'm guessing that the same glitch is in play.