If I was you, I'd order a 1760 barrel from Crosman ( #1760-001 quoted $12.08 today ) which is a 24 inch .177 barrel, then cut and crown 1/2 inch at a time until you see a decrease in average velocity, then stop.
Make sure your bolt probe is long enough so that the pellet skirt completely clears the transfer port, but no longer. ( IMHO )
Any valve mod is likely to increase CO2 usage, but there are mods you can do to reduce it. ( at expense of velocity/power )
With your criteria of not increasing CO2 usage, I'm pretty sure there's not much else you can do.
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