I visited an exhaust shop couple days ago,
sincemy car is single turbo (535i), I am going to have 3.5 inch SINGLE pipe all the way out to rear diff area, and y-3" pipe it to dual vibrant mufflers and wual tip set-up.
there is no reason to do dual pipe all the way, because if you look under the car, there is only about 3.5" down pipe from factory, and then they dual it out...
again, this is only for 535i,
550i, true dual...all the way.