If you get the right gas ram, for the gun, it should take it up to the legal limit. If you get the gun serviced it will run at about 10ft/lbs. You won't get a massive power gain by going for gas ram, and it will be cheaper having the gun serviced.
Anything over 12ft/lbs willl require a firearms certificate, and no gun shop will tune your gun, over the legal limit, without one.