I know on the SN95 platform the MACs need a MAC specific mid pipe. Not sure about the S197s, call MAC and ask or email them. That said its still going to sound like that with a H pipe. The Roush mufflers are notorious for this sort of thing, I'd swap the mufflers out before I did the prochamber. Throw some Magnaflows on there and profit.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.