Akrapovič list a
slip-on for the 2008 to 2015 1400GTR and C14 in its German site, meaning that it's TÜV approved, meaning that it's road legal as good as everywhere. The page even shows off the EC, EPA, CARB and ECE approval. I just don't know what it costs, and I don't believe the
5.1 HP increase they claim. The curve seems strange, though, as I don't remember having seen the drop at 2500 rpm, but I think all the diagrams I've seen start at 3000 rpm, like
this one.
To Akrapovič's credit, I read once a report on 11 slip on's being sold in Germany for a Bandit or some other popular bike, where the bike was dyno'ed before and after every change. Every manufacturer claimed stupid numbers, but in fact the changes were negligible, less than 1/2 hp for most, sometimes even in the minus. Remus did fulfill its promised 1 HP gain, and Akrapovič 2.5 HP gain.