A little lower velocity, maybe, although shotguns are not the same as rifles or handguns, they use a different sort of powder, and are lower velocity to begin with.
Secondly, spread is not affected to any detectable degree by barrel length, but rather by the amount of constriction, or lack thereof, in the muzzle end of the barrel. It's called choke. A long barrel with a cylinder choke will pattern pretty much the same as a shorter barrel with the same choke. (That's as open as you can get, it takes a special spreader barrel, sort of like a blunderbuss (the ancestor of the shotgun).
If anything the longer barrel might give more spread, because of it's slightly higher velocity with the same load. (And it would depend on the load, low base target loads would likely show the least difference, with higher speed, and higher recoiling, hunting or defense loads showing a bigger variation in velocity and spread. In any event the difference would be very small, as it's a secondary effect. The primary effect is the choke.
We're talking about a sawed-off shotgun here and you're talking about a choke?
Take the same shot shell and fire it through a 20" barrel and a 16" barrel and there's a big difference. Cool it with your equivocating with chokes and loads and powders.
Geez Louise.
I'm more interested in your answer to my post #1122.