i have never owned a six speed but i have rode a bike with one. i did a 5,000 mile trip (almost all interstate speeds) and the five speed trani did just fine. i think a six speed would work better only for the high speeds. i have no problems with the 5 speed and it has over 41,000 miles on it
I agree with your thinking. In order for the 6 speed to be "comfortable" you need at least 65-70 MPH before shifting into 6th gear so the engine RPM's aren't too low.