Buy a Hipac, pin the valve and cut off the piercing pin. Use a heavier hammer spring. These mods alone will get you 500+ fps on a stock barrel 2240 with 14.3 grain pellets at a 2000 psi fill. In my experience every 6 inches you add to the barrel will get you another 100 fps.Here's my Hipac modded gun. I did the above mentioned mods plus bored out the valve slightly. 12" barrel slings JSB exact 15.9 grain pellets at 650 fps.
CO2 is used because at normal temperatures a large percentage of it is a liquid, so a lot more of it can be packed in to a 12g. sized container.... and that means more shots.... There aren't that many other non-flammable gasses that can be stored at room temperature as a liquid (I can't think of any offhand).... It's that boiling off to change from a liquid to a gas that causes the drop in temperature when firing rapidly.... and also that causes the lower vapour pressure (and lower velocity) in cold conditions....
I'd like to hear about these "several strengths of gas" used in airsoft.... please educate us.... I've heard of "green gas" (which is Propane) that is ignited for each shot, of course.... What else is there besides HPA where you can vary the pressure of the regulator setpoint?....
CO2 - different to all the others in that you can't inject it into normal GBB mags, but it must be bought in 12g capsules which are inserted into special magazines. Very powerful, the only gas that's really happy in the cold but normally requires at least a metal slide to not shred your gun.