I'll echo what Albiscuit said.
Don't worry about it for now.
Out the box, the CM16 will perform fine for a very long time.
The G&G hop up rubbers are very good so any upgrade there could only lead to tiny improvements.
The gearboxes are fine so don't go tinkering in there.
And the spring and Barrel are good, don't touch them.
Use the money you'd spend on upgrades on a better gun.
Say you spent (rough prices from the top of my head)
£45 - tightbore barrel
£20 - Prowin hop up
£12 - prommy purple rubber
£15 - spring
£30 - high Torque motor
£10 wiring to deans plug
You're already halfway to the price of a better gun! And over the cost of the initial gun just on upgrades that won't make much difference.
It sounds boring I know, as tinkering with your gun between games is a lot of fun.
But I'm willing to bet your gun works fine and you're happy with how it performs, so leave it that way