anything that causes turbulence in the exhaust flow will slow it down. even a gutted cat or glasspack will harm flow. this isn't rocket science Jr. here is a cutaway of a glasspack, every one of those little scoops is going to slow down exhaust flow. i'm not saying our going to lose 25hp from a glasspack, but is will harm performance.
i understand that the OEM exhaust is "good enough" for 350hp, but a better flowing one will BETTER performance than the stock one. if you honestly think that a more restrictive exhaust is not going to harm performance, your a moron.