What are your power goals?
Stock ITR mufflers have dynoed maxed out around 250whp. I've been considering making my own oem style muffler but with 3" internals. I want something that look stock, is relatively quiet and will support 330-350whp maxed out. I may just give in and put something straight through on the car.
I put a dynomax 3" truck muffler on my civic. On v8s they have made over 600hp, so 1 should flow enough for 300hp. It was quiet but I couldn't stand the look of it. Looked like a wheelchair van.
Currently running stock eg muffler with like 1" diameter outlet haha.