jhautz has it right. The C rating by itself doesn't tell you much. Multiply the C rating by the mAh capacity/1000 to get the output current rating. So 20C x 5000mAh/1000 = 100 amps constant output. This seems
too low for a flux IMO. You could try punch control etc but with Turnigy\Flightmax packs being so cheap just buy some packs with better current output instead of risking it.