Just be mindful that in many cases money can be spent, the end result being your bike makes more noise, sounds cool, runs nice, possibly uses slightly more fuel, but does not make any more power. The reason ? BMW are known for getting it right regarding the balance between quiet operation and maximum power output. Going back to the K100 as an example of the first in the K series, there were numerous after-market exhausts made for them, all dyno tested and compared against the original stock unit,................. none made any more power overall, but almost all were noisier in varying degrees. Those exhausts did have 1 benefit though, they were all lighter than stock. Motad for example, saved 4.5 kg's, I know, I had one. Leave it stock and enjoy it. If you want more power, buy a K1600GTL perhaps. My opinion, others may differ in their views of course.