I am pretty certain that a 28 would do well on ducks based on my experience with the .410.
I have a TriStar G2 .410 as the backup to my normal 20.
My non-scientific test was using the gun on a sea duck hunt in SF Bay.
I shot a combination of steel 6s, bismuth 4s, and tungsten 9s.
Tungsten was obviously the best. I could confidently cartwheel scoters at the outer edge of the decoys, which was maybe a 40 yard shot? If price were no object, a 410 with tss would kill anything I am likely to shoot at.
Bismuth performed down the middle. I had a bit less killing ability than with the tungsten, but I was still quite confident at all ranges.
Steel brought up the rear, mostly because there just are not that many steel pellets in a 410 shell. The holes in the pattern were obvious. I had to hold off to shoot until the ducks were very close, maybe within 20 yards. This is not to say that there was not enough energy in the pellets, just not enough pellets. I killed a cripple that was swimming outside the decoys with steel, but it was clearly a lucky shot. More often, they kept swimming.
Bumping up to a 28 would have worked fine.