C ratings are a bunch of bull since there is no testing standard of any kind so they can put whatever number they want on the label.

That being said, better batteries ABSOLUTELY DO make a difference. Yes, a lower mah, lower C battery has plenty of power to supply the motor and propel the car but a better one will see the voltage drop less under load. In other words, MORE POWER. 21.5 is such a slow and even class that even slight gains in power make a big difference in finishing positions. From what I can see, you are behind the curve in both the motor and battery area.