Oh, those poor, poor dolphins !
heh . . just kidding . .
seriously tho, was the car running OK (well as well as it would run with a blocked cat anyway ) before you modded the cat ?
Did you mod anything else at the same time ?
I think Ignition might have the best plan - If you can try another one and it sorts it out, perhaps grab one from the wreckers. Like you say, it might not be the most environmentally sound cat, but it'll possibly solve the banging problem ( so you can drive the car ), hopefully won't be blocked ( so you can drive the car fast
) and won't leave you in the poo if the epa / rta or whoever has a look underneath . . .