how gentle is the piston pressure (ie how tight is the seal) ?
It's pretty hard to get a bad shooting 25mm 77, so I'm worried the sealing is suspect...
Also try chopping a coil off the spring (without removing the piston and disturbing as little as possible). If power goes up or stays the same, it's over sprung... find a more suitable spring. How thick is the wire ?
Finally, maybe worth checking the "lump" hasn't drilled out the Tp to something silly.. 3.5-3.8 is fine, much more will be detrimental...
You need to fix it before you tune it...
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.