The reason is because the main restriction in the catback system is the cat, and the u-bend. Not the catback it's self.
And the next restriction is the stock mufflers. Most everyone including me hates the sound of aftermarket mufflers on these cars. Therfore they just leave the stock mufflers on.
And if your going to leave the stock mufflers on then there is no point in changing the catback out.
Another reason is we have plenty of people including my self that have been in the low 12's and 11's trapping 107 - 115's in the 1/4 on the stock catback.
