Yes I know the BMW NineT hasn't been released yet but a couple of recent spy shots point to Arkrapovic testing new pieces in conjunction with BMW.
The double stack pipes here are most likely the stock equipment
while this one has a single pipe which looks like an Arkrapovic Silencer.
Also notice how the second picture of the NineT has kept the Concept Ninety's single seat while the other has a more traditional seat with back rack...
regardless of whichever exhaust that ends up with this boxer it ain't going to be loud as you guys think. These engine weren't known to be loud but has a distinct sound. So adding a exhuast would just give it a more pronouned boxer sound.