Flow

Can't increase the pressure, can only increase how much co2 get to the back of the pellet. CO2 that gets there "late" doesn't do much but make extra noise; past a certain point, squirting more co2 out isn't going to gain vel. Getting the co2 to the pellet smoother/faster will increase speed.

In .22, past 725fps or so, it's a matter of the little details. Smoother gas passages, better valving will earn more gains that dumping more co2 per shot. That's one of the reasons lighter hammers seem to work better...a sharp "jab" at the valve seems to do more good (with less gas wasted) than a big slow round-house punch.

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To get the most power (but less speed) converting to .25 can get you into the 22-25 foot pound area. Basically, co2 LOVES a bigger bore (which is why .177's don't get the energy of .22's)....more bore volume, so more of that gas gets to the pellet base "right now" rather than being late to the party.