Play with it first then figure out what you need. No point in buying a car only to replace all the engine parts and turbo it if all you want is better handling.
BUT what I suggest you do first is get an M110 spring, remember 400FPS (or whatever your local limit is) is a LIMIT not a goal. No point in having it shoot 399.99 FPS when you test it only to have it shoot at 405 FPS the next day (making it illegal for play at the field in question).
Also might be a good idea to take it apart, shim and relube the gears with some high quality grease like Finish Line Teflon or something.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE