The R12GS has an engine speed and position sensor and a camshaft position sensor. Both are connected to the BMW-K. Neither are Hall effect devices. They are Variable Reluctance (VR) devices. Hmmm, it sounds like I am repeating myself... OH, I am! http://www.r1200gs.info/forum/12675-post3.html
If either were bad I'd expect more than the idle to be rough.