actually, you CAN upgrade spring guns, kinda. take it apart, grease up the tracks for the piston, grease the piston, shave things down so that pieces fit together better, maybe put a different spring in. ive done that to a few pistols, increased fps by about 50-75. but most everyone here is gonna say, why bother, its not worth the time.
and an mp5, be it spring or aeg, is not a sniper weapon. period.