john- That's not how it works. #8 shot loses its velocity, and hence loses its already low energy pretty quickly once it's flying through the air. #7.5 is a tad better, but it still loses its oomph at the same range no matter how much of it you have. Air resistance is significant, momentum for a little pellet is minimal.
2 oz. of #8 shot at 75 yards wouldn't hit any harder than 1 oz., since the pellets hit the bird one at a time.
What you really want is 1 1/8 oz. of #6 through a Full choke. That will be effective at a longer range. Actually, if you shoot #6 from a 20 Gauge, it will be, also. Or even #5.
There's a problem with that, though. #8 at 80 yards is a minor annoyance, whereas #6 at 80 yards will mess up anyone it hits. I know someone who got hit in the face at 80 yards with #6; his brother fired the shot. He had a few pellets embedded in his face, and one almost took out his eye. It ended up embedded in his eyelid.
Therefore, shot over #7.5 is NOT a good idea if you're not alone out there (seldom the case when dove hunting here). #8 is a better idea, if the area is crowded with people.