[Wicker Bill]

In my experience, a big waste.
I purchased a pre-owned MSC RS200. It came with standard 21,500 RM motor, inline configuration. Gearing had been changed to a lower gearing (higher numerically).
At the time I only had a short routed track. Car was an unbelievable handful.
I put in an M3 motor, 17,000 RPM, and went back to stock gearing of 9:27. It made the car driveable.

Stick with the Sprinter Junior motor. At 18K it might be too much, but give it a try. Futz around with gearing before changing to a faster motor.

My two cents worth,
David