It depends on how good the air seals are in the ARES stock parts, and how strong the cylinder, cylinder head and piston are.
If the air seals are good, then they can probably be further improved with PTFE tape. If the cylinder, cylinder head and piston can all withstand ~500 FPS, and the bolt pull is smooth and relatively easy, and the piston moves easily inside the cylinder. Then it's probably not worth dumping £120+ into those parts just yet.
The price for them isn't too bad though, I spent about £220 on Laylax cylinder, cylinder head, piston, piston head, spring, spring guide.
What FPS is it shooting at currently, and with what consistency (too much effort to check back in this thread to find out)?
Also, what parts have you changed so far in the gun, if any?