I found a company that makes 63.5mm and 76mm diameter high flow cats. My question for any exhaust flow experts out there is which should I get considering my exhaust is 70mm? I guess 63.5mm would just extend the the same diameter as the collector a bit longer, but what's the point of having a 70mm exhaust if you have a piece right before it that is smaller?

 

The gains would be minute when comparing these two sizes. One would just be a higher flowing portion of the original size and the other would be even better flowing portion than its counterpart but restricts where it mates to the exhaust. It would be a safe bet that the bigger cat would provide better gains.