Originally Posted by Doug007
2x 2.5" diameter pipes has an area that is about equivalent to a single 3.5" pipe.
I see your point about clearance. You could always try to oval the pipe a bit to maintain the same area with a smaller vertical dimension. As long as it's not too extreme, it won't have a huge effect on backpressure. It's primarily a question of flow area.
The biggest gain will be just replacing/removing that huge catalytic converter. The pipe sizes/arrangements past that will be insignificant compared to a high flow cat or none at all.
What cat? I know on the 335i n55 there are no cats or res on the pipes...