Well, now, I too had that 28 bore itch.
Then did some research and actually looked at auction websites where the weight of these is given against the maker's 20 bore of the same time type. So in reality you are comparing, say, Ugartechea with AYA or such. Subject to the length of stock and density of the wood a SBS 20 bore in 70mm will wight from 5lbs 8ozs to 5lbs 14ozs. A SBS 28 bore in 70mm about a mere five ounces less like for like.
Truth is there's little real weight saving and a lot more utility for that loss of any supposed benefit or a "lighter" gun. IMHO a light loaded 20 bore with the old classic 2 1/2" loading of 13/16 ounce will outperform a souped up 2 3/4" 28 bore with today's 23 gram load even though 13/16 ounce and 23 grams are the same payload. And if and when non-toxic becomes mandatory that 20 bore be far less costly to feed.
I am of the LAMPALOT school and, yes, acquired an AYA No4 in .410" which I will use with 2 1/2" cartridges such as the excellent Eley Trap for practice and, maybe Eley 3" Subsonic for live quarry.