Here's a list of most of the upgrades and what they do :
Barrels - Very very good upgrade
Volumizers - Barely anything , the 1 thing they actually do is let you have more volume in LPR which doesn't actually do anything.
Bolts - Can help or hurt , get a unmatched bolt to your gun , and it can ruin the gun , get a matched bolt and it'll make gun get more shots per tank.
Triggers - They do something , its mainly in prefrence , try em out and see what you think is more comfortable.
ERA (freeflow mod ) - This is a must have , everyone i've talked to that has got it loves thiers.
LP internals - Does very little , but i've heard if you use LP internals ( valve ) + freeflow your gun works great.
Thats a few..........