As long as you have a pair of cats upstream of the rear o2 sensors you will be fine. Aftermarket catted pipes all have 2 cats and they are just fine. The 6 cats on the stock pipe is beyond overkill.
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 ****.