To Karlp : the R12GS engine has it's highest torque output at around 5500/5750 rpm, (<'08 / >'08) which means that's where it's strongest, and that's where the engine turns at it's best, where it's most comfortable. Should therefore be no problem riding between 5000 and 6000 rpm. The R12GS has it's max power output at around 7000/7500 rpm. Max engine speed is at around 8000, so it's useless going higher than that.
Note : I've written 'around X rpm' because dyno results always show a different figure compared with the official ones.
Wanting to find out what it's capable of, I've gone full out once on an empty unrestricted speed piece of Autobahn, for a distance of about 30km. The speedometer went to 220kph which is the R12GS's official max speed. Speedometers are always a bit 'optimistic', but the speed was impressive. When I got sudden drawbacks in power, then re-accelerations, etc, I realised the revlimiter was cutting in. Which, to be honest, didn't feel comfortable at that speed.
As far as long term effects of high speeding goes : I've read some stories of RH exhaust valve mishaps at high speeds, but unfortunately only BMW knows enough about the cause of RH exhaust valve failure to make conclusions.
On long stretches of highway, traffic permitting, 5 to 6000 rpm is where I ride.
