The following was sent to me by RobSmyth last year in response to a similar query. I have not done it yet. I have a spare valve assembly - the black cylindrical thing = but the threading is suspect so a friend is sorting it out.
"-Remove the air cylinder.
-Remove the black valve cover that the cylinder mates up to (you will remove either 2 or 3 cap screws with an allen wrench).
-If memory serves here, once the valve cover is off you will see some Belleville washers (cone shaped) stacked alternating end to end. I was told to turn a few of these washers around to increase the length of these stacked together. However, mine were already as long as they would go. After digging through my left-over-hardware pail I found a thicker flatwasher that I replaced one of these Bellevilles with. It ended up being only .016 longer but made a big difference. That .016" upped the speed to 525fps with CPLs.
I never tried to go further and have not shot it over the chrony to look at shot count yet. I really need to do that. But it is indeed a very easy change to do. Good luck
Shot count - easily a hundred good ones. Breech to end barrel already 8 inches. Free floating barrel - would not take weight of HW silencer also no room as barrel close to air cylinder - quiet discharge anyway. Red dot sight mounted on rail available as accessory = a potent combination. Get one with ergonomic grip if you get one. This is not a roustabout weapon. It is a delicate piece of kit relatively speaking. Not a Steyr or Morini or whatever but adequate from novice precision plinking and target. Although it has a 8 shot cylinder I do not recommend rapid cycling as the trigger needs to be fully released before squeezing again. Getting over excited with rapid fire can jam it or cause valve to stick and discharge cylinder.