Agree on sealing, I machine a taper to the end of the sleave, so that the outer edge is about 0.3mm further back than the inner edge.. the void is filled with loctite / bond and it ensures that the inside bore is really tight flush to the end of the comp tube. On a TX, where the endcap is remoaveable, I machine a recess around the outside of the face face such that the unmachined inner of the end cap protrudes slightly into the sleave (around 1mm), which makes for a really good seal.
As for your 21, just stick with 22 or 23, you have so much more scope for tuning... quite why it's short on power, no idea besides what you have suggested, but that won't stop me guessing
- surface finish of the bore holding lube or not (which provides a little power through combustion)
- pellet seat / fit in the breech affecting release pressure
- leaks around the tp and the end plug