4000mah 10C cells will be able to supply 40amps (4000mA X 10). 4000mAh 15C cells will be able to supply 60amps (4000mA X 15). The SV setup draws around 40 amps depending upon prop and voltage. Using a higher C rating battery allows you to draw more amps from the battery and stay within the safe operating range of the battery. A higher C rating pack will hold its voltage better and have a longer life. Using packs to their limit of current rating shortens their life. The best packs have C ratings around 30.