Won't be a ton louder. I would not worry about it. Although from your other thread with the mod list where you have ported heads and cams etc... the stock pipe is going to be a restriction at 2.25" diameter. Most aftermarket pipes are 2.5" instead.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.