which battery pack would i need to use with an eflite 400 motor, 6.6:1 gearbox and a 12x6 prop???

i'm thinking i would need a high discharge (15C) 2000mah, 3 cell lipo.

is this right???

what would happen if i use my 1500 mah, 8C, 3 cell lipo??

thanks
tony