I don't think velocity is the only limiting factor, but 40-45 yds for me. Bigger ducks closer to 40. If it's a ripping wind, closer yet. I don't like taking first shots at birds over 40 plus a little. If you are confident of your pattern and your shooting, then you are probably good to go.
The Boss shells have worked very well for me on pheasants, so far (#5s). But I have not patterned them. They seem to show less penetration than the Kents, and they should - given that the Kents are starting out about 75 fps faster. But that's just subjective estimation from cleaning birds. I did have on bird that popped out an unplated pellet. He did not show signs of a festering older wound, but that's probably the explanation.
I don't know what my bp loads are doing for velocity but I would guess that they are in the 1250 range.