I don't see how it would affect accuracy all that much, sure the pattern may be a tad larger (even twice as large as a 28 inch barrel is quite suitable for most hunting situations) than those from a longer barrel. As for velocity, I seem to recall an article in an American gun rag a few years ago that mentioned that shotguns attain their maximum velocities in the first 12 to 16? inches of barrel, anything longer doesn't affect velocities significantly but it does help with "pointability", which is something that either ghost ring sights or a red dot would fix quite nicely.
As for pattern sizes, I guess there's only one way to find out, screw in a full choke, then a modified choke and lastly a cylinder bore to a 14, an 18, a 20, and finally a 28 inch barrel, and proceed to blast away at a target at a set distance, someone mentioned 40 yards above, and see just how much shot dispersion is <really> affected, my money is on "not significantly".