The piston gets cocked back by the gears then released, so the speed of gun the won't have any effect on fps until you start running into timing issues and losing compression.
Depending what gun you have and what fps you want to be at, you may have to change out quite a lot, or nothing at all.
It's not necessary to be over 400-420fps, or 1.5-1.6j (muzzle energy is the actual LIMIT for airsoft fields, the fps is just an inaccurate reference).
At ~400fps you'll be in the 260-300ft range with a fair bit of accuracy depending on your upgrades. Doesn't leave much to be wanted.
And we've got highly varying weights of ammo in airsoft, so when WE say "x fps" we are referring to it's speed on a .20g BB. Heavier ammo will be shot at a slower speed, but have the same, a little bit less, or up to a shitload more muzzle energy.
It's something to be aware of if you're upgrading your gun yourself.
You could chrono in under field limits using .20s, but actually be well over the field limit using .30s.