I think this is something that should be looked into, I'd like to see a test with barrels with different control bore lengths between markers with different operating pressures. A mag may be a confusing example, because of it's lower breech pressure, but a test between a Tippmann and a Droid/Geo/Quest/(name your lp spoolie) would be interesting.
I don't know that we actually have breech pressures for too many modern guns. Tom Kaye did a bunch - but that was back a while ago. The problem with the test is that it involves cutting into guns - so it would be really, really expensive. And operating pressure doesn't necessarily have much to do with breech pressure. In the old test (this is off the top of my head - so I might be a bit off) - the angel was the highest breech pressure at 110 psi or so. the Mag was at the lower end at about 60 psi - with a much more gradual power curve - while the cocker was in the middle at 80-ish.
It's been a while so I may be off on those - but I'm pretty certain about the mag and angel - they were sorta the point of the test - TK was showing that operating pressure isn't the same as breech pressure.
but yes, threre should be some relationship between optimal control bore length and breech pressure - the higher the shorter.
The other possibility is that there is something else at work here. Paintballs accelerate really rapidly - a matter of just a hand-full of milliseconds. It's possible that the optimal lengths aren't different enough to matter. I'm totally making these numbers up - so don't take them as some sort of endorsement - but it's possible that the lowest breech pressure of, say 50 psi and the highest at 110 - a ratio of more than 2:1 - might only have a .5" difference in optimal length. say a 10" control bore and a 9.5" control bore.
Since the pressure drops so quickly behind the ball as it moves - the starting pressure will start to even out. so, let's take that same example, 50 and 110 psi.
after the volume behind the ball has doubled now we're looking at 25 and 55 psi, double that again and we're at 12.5 and 22.5. double it again and we're at 6.25 and 11.25. so now we're still at a ratio of 2:1 - but the psi differences are tiny - only 5 psi at this point - so it's possible that the only significant difference goes away so quickly that the optimal acceleration length just isn't that different.