The valve is made to really limit the amount of co2 it spits out per shot (which is why they shoot in the 400's but get like 70-80 shots per 12gr.).
Besides a light striker and light striker spring, they used a couple of tricks.
First trick is a tiny transfer port hole in the valve...but there is also a transfer port hole in the air tube leading up to the barrel. The barrel is full length soldiered on and there is no easy access from below, so that mod requires some creative thinking (and I'd just leave it alone and accept the 100fps increase possible without doing that).
Second trick is that the valve stem is not the same as a 22XX stem although they will interhange). The valve stem head is wider...which makes for a smaller gap between in and the inside of the valve...which limits gas flow. They also used a longer shank on the valve stem, so that it will impact the end of the valve and not alow the valve to open any farther (will hunt up a picture of the valve stem).
2000 valve stem to the left, 2240 stem to the right. Can see that I already reduced the diameter of the valve stem head on the 2000...but didn't really pick up any speed. Notice how long the brass shank is leading to the piercing stem... it seems to be made to run into the back of the valve, limiting how far the valve will open (also limiting how big a hole in the 12gr....these guns tend to make pin-hole sized holes in 12gr's):
So if I were to hot rod one (and I'd rather not...have other guns, like this one as the sedate target gun it is), would increase the transfer port hole, swtich to a 2240 valve stem, and increase the striker spring.