I am currently using a 6900kV with a mamba max. The motor is crazy fast already, and I am still only using a 20:61 ratio. However, you really have to be careful with this motor (buy extra spur and pinion gears).
The thnig I do not like is how if you are using a brushless motor and you don't have something tightened just right (like the grub screw) then you will pay. I have gone through 1 1/2 spur gears due to minor errors in adjustment and checking to make sure everything was tight.
And personally I find that these faster motors bring out the imperfections in your chassis. I got mine second hand, and there are some screws missing, and the rear gearbox stiffener (locks the spur gear cover to the rear assembly) is gone on mine. So every time my chassis flexes a little bit, the drivetrain grinds a bit.
I think you COULD put a 7700 in there... but it' suicide if you don't have the drivetrain and everything loctite-ed down. But why not go for it anyways?
