I'm not saying that you need to run a higher voltage, but that a couple of 7.4V 30C 3600s will be fine for testing if you already have the packs.
Higher voltages paired with a lower kV motor and appropriate gearing will be more efficient than a lower voltage with a higher kV motor geared to the same speed. This is because power = voltage * current. If the power stays the same (same top speed), and voltage increases and therefore current falls. Current causes heat build up in wires and connectors etc, (which is why connectors can cope with a huge amount of voltage, but not that much current). Additional heat causes the resistance to rise, and thus even more energy is wasted as heat. (Vicious circle).
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