[why?]

Originally Posted by RZNT4R

Well, the way I see it, if I want a one piece carbon driveshaft, I paid for a useless stock steel driveshaft on top of it. If i want a backseat delete, i paid for those for nothing, if I want shocks and springs, I've got a set of useless ones under the car now that cost a good chunk of change at the factory. Same with a lighter Ti catback.

Not having parts duplication is always cheaper, the question would be, what kind of markup would that added performance warrant? I'd wager paying for the upcharge would beat the price to do it after the fact.

You can sell all that stuff. If the manufacturer did it you would need to pay double.