I have hunted birds with 20 inch barrels before. And it's not like that's a considerable longer length than 18.5"s.
As Mike has pointed out, one advantage to the longer barrel is the longer sight radius and how it feels as it's swinging. Longer barrels do allow a little more precise "bead".
But as far as how the shell performs, isn't any more meaningful with the longer barrel than the short one and I think that is at the root of your question.
Several of us have discussed this at length before, but the short answer is, way back in the day before "smokeless" powder was created when black powder was still widely in use, longer barrels were more beneficial than shorter ones due to longer range because the longer barrel allowed the pressure to build more as it was being fired.
Smokeless powder that is used today however, for the most part, has already burned out relatively fast in the bore and doesn't really see much of an increase in velocity regardless of barrel length, so the velocity ratings on the shell are going to more consistent whether it's an 18 inch barrel, or a 32 inch barrel.
In my honest and humble personal opinion, depending on the max distance that your predetermined distance where you consider a bird out of range, a bigger consideration than barrel length is your choke and shot size and how it patterns.
It is absolutely possible to find a great combination of choke and shot to use a shorter barrel that is just as good as a long one.
And since you may have to play around and use different chokes to find the sweet spot, having a shorter barrel that accepts screw in chokes would be preferred obviously.
There is a lot of choke testing on this forum. I have personally seen where one brand of choke outperforms another brand of choke of the identical constriction from counting pellets on target. And only testing in your specific barrel is going to be the biggest factor.