You need to understand that TC barrels are measured for their overall length. So the 20Ga Pro Hunter barrels are 28" long but you have subtract 3" for the chamber.
That's the way all barrels are measured regardless if they are shotgun, rimfire, centerfire, bolt action, single shot, over under, etc...
A longer 28 vs. 26 inch barrel in a shotgun isn't going to add any velocity at all. In fact, a shotgun will burnall of its powder in less than 24 inches.
For a slug gun I would take the shortest barrel possible for the encore. A shorter barrel is more stiff than a longer barrel and as suchis a safer betto be the more accurate barrel.