over here where we have to "certify" engine swaps in our cars we pretty much have to have a cat in the exhaust or it wont pass the cert. Best way around that (all my mates have done it) is to smash the shit out of the inside of the cat and then its just a straight thru pipe pretty much. But in your case where your getting a new system made, just get one of those "high flow" cats, then if the law ever comes knocking your safe.
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