The 2-1 exhausts from SR-racing are quite popular over here in Germany. http://www.sr-racing.de/anlagen/BMW/R100GS.jpg No one has ever complained about performance, in fact they give a rather significant boost over the complete rpm range. However, I would not use them if classic looks were important. Regarding the second crossover tube of the post '81 models: I have a technical article where the authors (engineers from zeuna staerker, the company that developed and manufactured the BMW exhausts then) describe the effect of the crossovers as follows: - The first crossover improves low-mid rpm torque - The second crossover improves mid-top rpm torque. My dyno runs have shown this exactly.