Certainly I'm not knowledgeable enough to answer that stuff... At the end of the day a trip to the rolling road before and after tells no lies (as long as it's the same rolling road!) .... As for 'why don't OEM do it?'... You could say the same about the gains obtained from a decent aftermarket exhaust system, when if they were fitted to all cars at the factory en masse the cost difference would be minimal. Manufacturers move in mysterious ways!!
K
