Bajie

With all this downpipe additions going on I have a question before I take out my junior hack-saw:
HKS Super Drager decatted exhaust.
I want to buy a downpipe from a different manufacturer.
You cut an inch, or two off your current exhaust so the downpipe will mate.
Drive off burbling, popping and frying BMWs.
Your M.O.T comes up and you fail on emissions.
I can't find an HKS twin dump downpipe.
Do I need one. Am I caught up in the hype.