Bruce- one of the great things about history, whether aviation, the law, or our beloved doubles afield, is the variety of opinions, and no one man can be an expert on all phases of shotgunnery. I won't speak for the small bore fans, the die-hards who pursue Sir Ruff through class three jungle canopy in early season, or the gents who love shore birds and snipe, or can afford driven birds "across the pond"-- Shotgunnery, like golf, has many facets and the "game always wins in the end"--
That being said, I have 50 years as a "die-hard" waterfowler, with some pheasant hunting experiences too involving steel and non-toxic shot. I applaud the members who thrill at a well centered scaup or bufflehead in a late November squall over a big water spread, and if they choose to use a DHE 12 rather than a Binelli or a Mossberg, A-OK with me. The premium shells are a "drop in the bucket" compared to the cost or replacing bulged or damaged barrels in your Parker, Ithaca, Fox or Smith.
I have used the Hevi-Shot for classic doubles 12- 1& 1/8 oz. no. 5 in both my Smith 2E with 32" Nitro barrels, and my Parker 12 GHE with 28" Vulcan barrels, and they kill mallards (if you point, swing and follow through) but I use either of those doubles on "special occasions"- When the weather is foul and nasty and a big Norther starts moving those big redleg mallards into our area, I use the "evil black shotgun" the 835 Mossberg pump with 28" ribbed ported barrel and the Carlson tubes I won at a DU Sponsor event some years ago, with Federal 3" steel shot.
I have heard very good things about ALL the non-toxic and custom shell makers others have detailed on this thread- In the final analysis, I would suggest having the Parker (or other older quality double) you are thinking of using for waterfowling checked over first by a competent double gunsmith, and if he happens to be a waterfowler, as is my gunsmith/friend Brad Bachelder, asks him for his shotshell recommendation for your particular gun.
I am pleased you are on my PGCA "friends list", I have both enjoyed and learned from all your posts herein, as I have from others as well, so I ask you to please reconsider leaving the forum--