[Nonfractal]

I do use king 30s inline in several cars.
What I would say is that you might be thinking that the braking is poor because the cars don't stop as quickly as your slower cars.but decelerating from10 metres per second or more is a big challenge for all of the components of a car. 

Shorten your gearing (see my "gearing explained" thread. )
Get yourself a better controller to handle deceleration from high speeds (see My "braking" thread.)
Shed weight from the car 
Clean the motor.
Ensure your tyres are true.

AlanW