Some pattern-part exhaust manufacturers use some wild n wonderful silencer/muffler internals that can really strangle the engine. I bought (in times of severe hardship) a Euroflow rear box once, it hissed really loudly under power and made the car feel strained. Unlike the better boxes, this one had a capped-off inlet pipe with holes in the tube walls! Not unusual technology, but not used like that... It stopped the sound pulses coming down the pipe sure, but created extra noise instead whilst inhibiting the 'draw' through the exhaust.
You dont get silencers/mufflers before the cat, they come after.
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