Today I noticed something really interesting:
As I mentioned before, the two exhaust pipes take slightly different routes after the X-pipe. In particular, the outer pipe, connected to the rear turbo with the shorter downpipe, has a greater length.
Roughly measuring the pipe length difference from this
picture, it appears to be around 15-20 cm (6-8 inches), VERY similar to the downpipes differnce.
Could it be that BMW initially designed an equal-length exhaust but ultimately decided to place the X-pipe in the middle ruining it for other reasons?
It seems like replacing the X-pipe with two straight tubes, then merging the pipes where the rear muffler would reside (either with an X-pipe or a single-exit merge) could potentially perfectly equalize the sound.
Like this:
