RE: Question
Swampcollie, it sounds like you've got a little experience with the Hevi-shot. Have you measured the "duck" loads yourself? If so how did the density compare to Lead or Hevi-Steel? I only shoot the Hevi-Shot goose loads and found them to be the real deal, and I'm pretty sure they load 20ga "goose" loads.
Also, don't overlook hevi-steel, I'm pretty sure they load it in 20ga. While my experience wasn't that great it was due to a poor pattern from my gun, if it will pattern well for you then it is good stuff.
On a side note, we actually have a small guage only timber hunt every year. You can only hunt a double gun 20 guage or smaller on the hunt. I shoot a 28ga with Bismuth and it will shock folks how effective those loads are on a duck, you just don't have as much margin for error.