[Whiskey]

I personally would lose the stock piston head and cylinder head. The cylinder head will fail you, it's just a matter of time (they shatter). The piston head is solid aluminum with no ports with a decent air seal, but still an aluminum piston head is just a dumb idea either way you look at it...loose it. Also buy a C.A |Tappet plate as it could fail anytime. As for the piston, there actually a very well designed product and should last you years to come, as long as you have good AOE and not running an M150 etc.
I have the same gun and one word of advice is do not replace the air nozzle as it's a bit longer than anything I have found on the market. So far, trial and error have yielded very poor results for the Classic Army O-ring air nozzle, with a major air leak mating to the hopup unit. (currently the gun shoots a very consistant 360fps w/.28 BB's and with the C.A nozzle it shot 208fps w/ .2G BB's). I will be getting a hold of an aluminum SHS air nozzle that should be the same spec's as the KWA with the exception of an internal O-ring on the SHS. Also I have been looking into better a replacement for my current cylinder head, which is a C.A item. First the KWA cylinder head air nozzle is a fair bit longer than anything I have found to date, also the air seal was not ideal between the C.A cylinder head air nozzle and KWA air nozzle, until a fair bit or tuning was done. From what I have found SHS also makes an aluminum cylinder head with the extended air nozzle like the KWA. I will report back with my findings. If you are just looking for accuracy, adding a tight bore will not make a huge difference as factory they (KWA) have 6.05mm barrels. I went to a madbull 6.01mm ( it was cheap and my work stocks them lol) and it works great but you need to lose the 2GX bucking as it is different in diameter and will not seal well i the hopup chamber. Try the ARS hopup rubbers and an scs, you have a counter sniper weapon lol.
Best of luck.
-Ryan