Interesting information pete757. Agree about the OEM being a free-flow disaster and like your description of it as a "bagpipe".
There is so much science around exhaust headers - I had no idea but as Heinz suggested diving into the web does give hours of reading.....
In many ways driving it is all the evidence needed. I too am a happy camper.
Pete757 - you have come to a 4/4 from a plus 4 haven't you? I'd love to know the differences as you see them and how they compare - have you had the ECU remapped by the way?