Tactical Trijicon ACOG TA01 Style 4x32 Cross Sight Rifle Scope + Kill Flash
TROBB Solid Aluminium Dovetail Power Block with Gmac hollow probe and G-mac Bolt
Gmac Brass Power Adjuster Spring And Guide/Drilled Out Standard Valve For more air space
Hammer Mods//Gmac 30% Lighter valve Spring/Gmac Brass Safety Catch//Transfer port drilled out
Full Gmac Trigger kit/Gmac barrel cut to 9inch/Weihrauch Silencer/Custom Home Made Grips/
New TRACER LEDRAY GL2 LAMP 90m Range Beam//HI-Pac
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.