Josie, I thoroughly enjoyed reading your posts about buckshot, and just from reading them can see your experience with buckshot is extensive, and closely parallels mine. I hail from a state that until 1975 buckshot was the only legal implement to hunt deer with(New Jersey). Anyone who tells me that #1 buck out of a modified barrel will give you the best general results and hold a good pattern has obviously put in his time testing and patterning. I have always said that a gun that can't pattern 00 buck will often find #1 buck to be its saving grace. While I too am a slug user I have patterned many 12 gauge guns with buckshot and draw the same conclusions as you. Buckshot is really a very effective killer if used properly to the maximum ranges you mention, I think the biggest problem with wounded deer is that most people don't take the time to pattern their gun and have no idea what kind of pattern it throws. They just go to a store, buy a pack of 00 buck and figure they're good to go.
I do however also use #1 buck in smoothbore slug barrels on occasion. While we prefer slugs, we have a few places where in very thick cover, my son and I will do a one man push to the other, the stander using a slug, the pusher using #1 buck. As you say, doesn't hold a pattern real well out of a slug barrel because of the open choke, however ranges would never exceed 30 yards in our case, and we have found that we can put 4 out of the 16 pellets into an 8 X 8 inch circle at 30 yards consistently out of our guns, which will effectively kill a whitetail. But again, we've taken the time to pattern them, and know what they can, and more importantly, can't do !!!!
Garry