Dave, I've gone full circle on 20ga waterfowling. Deciding the 20 I'd used successfully for turkeys and deer somehow wasn't enough gun for ducks, even in those lead shot days, I stepped up first to a 16 but soon after to a 12. And when 3 1/2" 12s came along, I bought one of the early HK imported Benelli SBEs, which I still use more often than not for geese. But having shot thousands of 'em, I don't care if I ever run another 3 1/2" shell through it.
And the past two seasons have seen me doing virtually all of my duck hunting and a fair bit of goose shooting with a little 20ga I'd been toying with for my personal playtime waterfowling for the past several years. Have pretty well convinced myself that even for shooting behind our guest guns, the 20's handiness and low recoil offer advantages outweighing its smaller shot charge. And it's dang sure more fun to shoot than my 12s.
As for non-tox loads, I like everything about tungsten-matrix but its price, couldn't get confident with Bismuth, and was glad when my self-imposed ten box trial of Hevi-Shot was over and I could get back to hitting more close birds with steel. (HS patterns tight in my gun.) In fact, I've never managed to get mad at steel and actually finished this past season shooting 2 3/4" steel 4s, because they were the only 20s left in the shell room. Birds didn't seem to think I was under-gunned.