A cdi has several functions. From signals it receives from the trigger switch it tells the coil when to discharge its spark and for how long, adjusts timming and controls rpm's. This is called a map. A performance cdi does the same but with a different map. Your stock cdi advances timming just as the perf cdi does. The perf cdi just uses a different map for where and how much timming is used. There is only so much timming built into our engines so to say this cdi will advance timming more then stock is not correct. All the perf cdi does is apply the timming at different points as mapped. Todays stock engines will see very little if any bennefit from a performance cdi. Many have been sold that a no rev cdi will unleash all kinds of power, another falsehood. The stock GY6 powerband limit is around 7400 rpm . The stock cdi rev limit is 7800rpm. The engine has peaked the powerband at 7400rpm so pushing the engine above that rpm range only aids in premature wear and overheating of the engine. So is a cdi and hot coil an unnecessary item? When this engine was designed its components were designed for top efficiency. Now when one starts to mod an engine making it capable to suck more air and fuel then you will need an upgraded ignition system to aid in efficiently burning the added mixture. Also depending on how you build your engine will determine your power curve, and may require a cdi with a higher rev limit. The coil is the same thing. Stock coils are designed around a stock engine and work efficiently. Building your motor is the only way to make more power. True you will gain power by removing restrictions such in the air box and exhaust plus a rejet but these will be minimal gains at best but still a felt power increase. After that one has to build the engine so as to pull in and fill the cylinder more completely with a/f mixture, this is the time you will need to upgrade the ignition, for now you will need a stronger and longer spark to efficiently burn more fuel for more power. Now so I don't contradict myself, this hasn't always been the case for when these buggies first come to market the Chinese had taken a 125cc engine and made it into a 150cc engine plus they put them in a vehicle 200lbs. heavier then the scooters they were designed for. Since that time the epa has regulated the emissions which forced the chinese to use improved electrical designs more suited for the heavier buggy which was the main reason for going to the DC volt systems over the old AC volt systems.
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