If you run without magnets tuning becomes more important. With magnets defects like out of round tires, wobbly wheels, crooked axles and loose bearings tend to be masked. Adding aftermarket parts does not always make for a faster car. All of our cars use aftermarket parts, even ones that are good to start with like Slot.its and NSRs. One reason that we use aftermarket parts is for reliability, it can ruin your whole evening if a wheel falls off during a race. Another reason is flexibility of being able to try different gear ratios and tires. Finally there is the issue of consistency. You can count on a good brand of wheels being round and a good brand of gears having a smooth mesh.