While I don't quite understand how much of a difference milling the bolt stop does, it seems like milling out some of the rear wall would provide for greatly increased air flow. Hear me out (btw, I've searched for stuff like this and haven't found anything, so my apologies if this has already been posted)...
Ok there's the bolt stop with the rear wall behind it. Note the small hole where air passes through relative to the larger hole in the bolt stop.
Ok now here we have the rear wall (sorry for the picture quality). Not much space here for air to pass through (relatively speaking). Seems like if one were to mill/drill/dremel/bite out some more space in the rear wall to complement the larger gap in the bolt stop the air would flow easier hence possibly lowering pressure, raising efficiency...something like that. Better air flow usually means good things in paintball.
The only problems I can conceive:
1) You can't cut larger passages throughout the entire wall because there are those two little risers on which the rear cap presses. Therefore you'd have to align the rear wall and the bolt stop as best as possible each time you put in the bolt setup. Along with this, you'd need some sort of secure connection between the stop and wall to assure neither rotate against the other (how? who knows, but **** does happen) because I'm sure once the level of air flow changes one's velocity will too.
2) This isn't necessarily a "problem," but more of a concern. With this done, the back of the bolt, where there are a pair of o-rings, won't secure jack in the rear wall anymore. Therefore the air "chamber" will continue on past the rear wall at all times. So when the bolt fires forward the air will continually rush out until the bolt comes back. While not inherently bad, if one gets bolt stick and doesn't fix it fast then it theoretically could zap an entire tank of air.
I know there has to be some big thing wrong with what I'm conceiving otherwise someone would have done it already. Right?