I do not know the answer you are looking for... Then again, I doubt EM does either.
My understanding is that with Hevi-Metal, the shot sized is based on the steel shot, which should all be the same size...
The Hevi-Metal will vary in size and density depending on what size steel... So BB's will have larger hevi-shot slag of higher density than #4 Hevi-Metal will...
Now, looking at the slag Hevi-shot in a Hevi-Metal load, the slag varies greatly in size. Speaking to two different reps gave me two different answers. One rep told me that the density of the slag in a given shell is similar, while the other rep told me that the smaller slag is more dense so that all the shot in the shell has similar ballistics...
I have some friends that are avid waterfowl hunters who swear by the stuff. Having shot it myself, I could see no difference than my standard steel for ducks (if anything I have a preference for and more confidence with standard steel)... Having opened up some #3 loads, some of that slag is tiny (probably size #12 or so)... That did not give me any added confidence.
I will add that I think there could be a bit more benifit with geese though....
Now, how is that for not answering a question?