In the beginning of this video, about the 30 sec mark, you can hear it hit the rev limiter then the wind noise gets to be too loud. You're being conservative on the RPM's which you should be until it's broken in. The motor isn't straining, it was designed to run at high RPM's. It can be disconcerting to a new rider but it's normal. It slowly accelerates in 6th @50mph because you're not on the powerband. When I'm on the highway I don't shift to 6th until about 60mph.