Tight boar barrel and an air tight hop up. Both will gain you a goof few fps with the same spring. Will this be an indoor or out door gun?
If it's outdoor a 110 spring, extra long tight boar hidden under a silencer and a high torque motor. You will loose rate of fire but with a .25g bag you will punch through the bush.
I ran a mg36 with a 130 spring and that set up, it chopped groups down and I could out shoot most of the M4. But once the Systema came out it was out gunned though.
Indoor I would keep it stock and just swap for a high speed motor. As there is no point in anything else.
If you buy a GBB then run .30 and they shoot hot for indoor fps.
If you get angry at my post please re-read as a joke or sarcasm.