Decent hard wired MOSFET, hi torque motor, re wire with decent gauge wiring (most guns come with too thin a wire), replace your battery connectors with genuine Deans connectors and try to keep the number of connections in the path to a minimum. LiPo batteries are also a safe bet as they can deliver a higher surge current than NiMH batteries. If that's not snappy enough then you can start to look at lower ratio gears, dual sector gears and all that hi speed jazz (Sitting Duck can tell you ALLLLL about that - at length).