PCP or Hawglips would know alot better than me bill, but just guessing if a guy was shooting a little bigger shot like #7's and found the right choke, he could probably come up with something that would have enough pattern density to kill a turkey at 90-100 yards. The energy would be there at a hundred no doubt.
A full house load of straight 8's would probably kill a turkey pretty consistently at 75-80 yards.
I'm shooting a 20 gauge 1 5/8 oz load with one oz 9's and 5/8 oz 8's and though I've never shot at one that far, I'd say it would wad one up at 60 all day long.

This is just a guess on my part, but I'm probably pretty close and honestly probably leaning towards the conservative side. Preacher or especially Hawglips probably has some actual data to back up their theories.