I can sympathize with not wanting to start making mods to a new gun right away but shortening the spring is really the easiest way to make a substantial improvement in shot count. There are additional benefits too. Once it's not wasting so much CO2, your trajectory will be less susceptible to differences in ambient temperature, and it won't be as prone to self-cooling when you're shooting without a long pause between each shot.
There is also a completely reversible technique you a can try. A bstaley O-ring buffer. You simply put a stack of O-rings against the valve and they act as a bumper when the hammer flings forward to knock the valve open. When set up correctly, it alters the valve's lift and dwell and improves CO2 usage. Something like 3 or 4 -113 O-rings will put you in the ballpark, or 3 O-rings with some number of spacers (washers) to fine-tune the height of the stack.
The strategy is pretty simple and very similar to shortening the hammer spring. You just add O-rings until you see the velocity drop. However note that small changes in the stack height (or the hardness of the O-rings) can make a huge difference. For example, you might try a 3x stack and it has no effect and then you add a fourth and suddenly the velocity drops in half. Then it's a game of playing around with spacer washers or trying different O-ring durometers to find something that lets you retain, say, 90-95% of your original velocity. Very effective though once you get it figured out.