I have been told by an avid AirForce Texan shooter that the biggest gain without adding parts is to remove the valve return spring. He said this produces better results than high pressure bottles, extreme valves and stronger hammer springs. I have not tried it though as I am currently happy with my Texans performance but I am sure I will get to a point of trying it in the future.
Of course it goes without saying that before this is tried the bottle must be drained to zero pressure before removing the valve assembly.
Thurmond