first-ducks on the water, my thoughts are all the pellets skip off and knock the others out of the kill zone. try aimimg high when you have to finish them off, it works
second-I have not shot enough of the hevi shot type loads to say I'm an expert but will say that I think it does just what they say, but is that needed is my question. I kill most the ducks I shoot at with steel. I don't wait til they have there wings set and I will shoot at passing ducks. My criteria is that I'm shooting at the breast straight on or they are damn close.
If you're in an area that you can't be really choosy on shots then I would say it is worth the extra money, just to be sure you put them down hard.