My JG MP5 started out at about 360, then after about a thousand rounds settled nicely to a steady 340. Even though there's very little of the original JG gun left on it now, I still use the JG spring.
Until you can chrony the gun, I suggest you leave it alone if it's shooting fine. And once you chrony it, if it shoots 320 or better, leave it alone. It doesn't need to be upgraded.