I wouldn't bet against an open barrel and a mod barrel for columbari birds. A really quick shooter can have relatively short shots, as opposed to box birds, and may well wish for bigger patterns.

A 7# gun is heavy for a 20 bore, yes. But it is still 10% lighter than a 7 3/4# 12 bore. A person of small stature and/or lower physical strength might well find that to be the difference in being competitive. As has been pointed out to me several times, a 20 bore gun can have gripping areas more suitable for smaller hands, too. To assume that nobody would use a gun that was not a "real man's gun" is to forget that this is not the NFL and not all shooters are of heroic proportions.