I currently have a Greddy Turbo Kit, Stock Cat, & Greddy 2.25" SP exhaust. I would like to up grade my exhaust system to a 3" system.

Does this mean that, I have to get a high flow cat and/or test pipe that is 3", and a down pipe that is 3" to prevent bottle necking the exhaust flow, and so that the flanges bolt up correctly?

 

I run a downpipe that by-passes the stock cat location and acts as a test pipe, connecting to my Thermal 3inch exhaust.

 

well im sure you would see gains with just the 3" catback but a full 3" from the downpipe all the way to the muffler would increase performance the most. you would only be creating a "bottleneck" if you had like a 3" dp and a 2.5" exhaust. 2.5" dp to 3" exhaust would be ok though. btw a test pipe will probably also show some good gains over a high flow cat.