Will a high flow cat on an otherwise stock exhaust make any difference? The cat is OBD2 compliant.

 

Not really. It will only creat an air pocket. Your goal is to stream flow exhuast out the back and having the larger cat connected to the smaller exhuast piping would only create more back pressure.

Think of it as a straw that looks like this... ====(((((((====

You can only suck in as much "beer,water,whatever" as the end will let you. Same concept.