1) pellet leaves barrel sooner so recoil affects accuracy less
2) pellet already at max velocity 9inches down the barrel. Any more just slows it down.
3) shorter barrel will have less pronounced flexing because of higher resonant frequency of the shorter barrel.
4) shrouded barrel will even flex less due to the two different frequencies of the barrel and the shroud.
the only down side to a shorter barrel is less stability. Other than that Carbine barrels rule.
I have proof of that. As you've seen previously I cut my Norica Krono to 12 Inches, and all of a sudden it changed from being a complete dog to something half decent. I can get 20mm groupings at 30m with it, which is quite respectable for that rifle. Previously at that distance it sometimes missed the A4 sized target completely.
Keep us updated with what you do the new toy!