My goodness! People can go about barrels!
I think that because of the huge range of markers, players, techniques, and barrels the only thing we really know is that we don't know what the best combination is.
I know back players who use 10" barrels and have seen 20odd inch monsters as well. having said that, something worth noting is that 1 10" barrel is 10" shorter than a 20" barrel and thus has 10" less range (measured from the breach). That may sound trivial, but given the choice would you rather be 10" Closer or further away from your target?
My personal experience has lead me to believe that, for me 14" is the most consistent. This derives from a days play with a Freak kit on my cocker; I played with the 12" barrel for about half the day, and then switched fronts to the 14" in my opinion my shooting improved markedly, both on the field and just standing shooting at the chrony.
But that, as I'm sure you will agree, means diddly-squat in the grand scheme of things, people will shoot what they feel most comfortable with, and even if somebody did do some tests involving clamps and targets etc I don't think everyone would be convinced.
Richard