Easiest option? Move closer!
Harder option is delving into mods. I'm not 100% up to date with things due to both a dry spell in chairsofting and actual airsofting.
Assuming you have G&G M4 you have a pretty decent gun to work with. 160 quid is good enough to get a very good performing gun.
Gearbox work is pretty much inevitable sadly... Not much point having PDi barrels and r-hop if your still getting +/-15 FPS inconsistency.
So here are my suggestions:
O-ring nozzle
Double O-ring cylinder head
Polished cylinder
AOE correction on Piston(Optional)
Stretch O-ring on piston (Optional but recommended)
Good lubrication on the cylinder,that's a given.
Hop parts:
You can either go for mad custom hop set ups,like R-hop,flat hop,etc.. but I would honestly say standard hop stuff is perfectly fine and there's nothing inherently wrong with it. R-hop will get you some fantastic range and accuracy but it requires a lot of work and tuning. Besides,you have rate of fire to make up for a lot of things.
New barrel- Laylax,Madbull(Avoid the steel barrels,last I heard they had some QC issues. Coated aluminium ones are fine) and Lonex should be plenty fine. Don't go tighter than 6.03 unless you want more power. 6.05 is pretty popular.
For hop up units and buckings I'm sadly far out of date. G&G green stock ones are fine in my experience. Laylax purple have QC issues but are good. Lonex are popular too but that's it really.
For units,again,G&G make some of the better stock ones. Lonex make good aftermarket ones and I've heard good things about SHS.