hell yeah drill holes in a circular matter down the tube (exhaust swirls in the tube) then have it tack welded back.

that stuff in a bottle is nothing but a additive that cleans the carbon out of the motor. So use it like at least a week before the test. Test when the motor is warm, and DONT rev it up, or gun it before the test.

since that car requires a sniffer, focus on getting the cat fixed. I dont know where you live, but i know a guy here in snellvile that will do it cheap.