Originally Posted by maxnix
Not if you want ground clearance on the 1er!
Why BMW kept two 2.5 pipes coming out of the converter.
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.