I myself just got one of the same barrels. Only input ill offer is just my thoughts, not backing this up with any real info and am new to slug guns...
But i dont know how much more accurate you can get out of a shotgun slug gun. I just tried mine a couple times at 30 and 100 yds with open sites i was getting around 4" groups at 100 yds. Any advantage you get with an extra 6" of barrel on a shotgun would be minimal i would think. I doubt you would get any much more velocity, at least not enough to make a major difference shooting a slug at whatever you are shooting it at.
When i was after a shotgun barrel i was in it for something short with knock down power as I have some tac driving rifles and the big lever guns as well...
Like i said ive never shot some of the serious dedicated bolt slug guns that i know are very very accurate... but in my eyes slug guns are different animals altogether.