I built custom WGP Autococker paintball guns during the era when we switched from C02 to HPA (high pressure air). C02 is a pretty dense energy source. HPA provides a lot of pressure but a lower volume of gas. Some of the things we did was heavier hammers, opened up valves (both the valve stem/seat and the transfer port) etc. One of the more interesting things we found back then was a Nelson hammer spring kit that used thinner wire wound in a larger diameter pattern, which gave higher velocities with a seemingly lighter hammer strike.
HPA is simply head and shoulders better than C02 for shot to shot consistency.
I'm a hold out, and when I drag myself out to play once a year I'm still using C02, but I'll probably switch to an HPA setup pretty soon.