You will definately have to gear up with the sintered rotor to get the same top end performance that you got with your bonded rotor.
But just like the other people noted, the sintered rotor generates so much more torque that gearing up will not be a problem.
You WILL get better over-all performance out of your sintered rotor 8.5 than with the bonded rotor.
Usually when switching from the bonded rotor to the sintered rotor, most people gear up 2-3 teeth on the pinon (or more, but I would like my recommendations to be conservative).
With the sintered rotor you're going to get better torque, better brakes, better resistance to heat... meaning that as the system runs it won't be effected by heat as much as the bonded rotor will.
Eventually running the bonded rotor through heat cycle after heat cycle will result in it loosing most of it's magnetism, thus reducing torque, and efficiency...but producing more heat and as a by product of having less "magnetic resistance" more RPM. Overall power will be significantly less than that of sintered rotor.
You're on the right track with the sintered rotor, you just need to gear up!