Maybe you should wait until you get onto a computer to post, smartphones have made convergence possible for many people, but it will never beat a good old keyboard and mouse.
Anyway in answer to your question, why do people get stuff like high torque gears and motors? Each part has a different purpose, you'd use a high torque motor/gear setup to pull anything above an M120 spring. As was already said high speed gears and motors will increase rate of fire. Higher end pistons tend to last longer than the cheaper ones, especially if you are trying any of the aforementioned upgrades, i.e. more rps or more fps.
In answer to what upgrades to put in, it'd help if you tell us what your goal is, do you want more fps, rps, a more durable gearbox, more distance with your shots, or a quieter setup overall?