The issue with upgrades is you always seem to be needed one more thing - you end up with a completely different gun and a box of unused parts. Upgrade the gun, oh now got to upgrade the battery to compensate, oh need a full stock for the large battery, oh need a new body now, etc etc etc etc...anybody who started with a stock TM M4 would recall this process.
Actually I take that back about the battery part - back in my day it was NiCad only. These new LiPo's - wow...that changed the rules about AEG's and available power.
Still - having a stock gun in your inventory has plenty of merits - particularly for indoor games with stricter FPS limits. You never have to worry about not passing the chrony. If you really feel the need to upgrade something - get a new barrel before touching the internals. Upgrading from plastic to metal bushings is great advice and common sense - the plastic bushings are prone to cracking and yeild a broken gear in the process.
86 Tactics Change.