Owen,
I wouldn't set myself as a Pigeon expert, but i have shot Box birds about 20 times over the years, and with some very good shots. I knew of nobody who shot extra full first, and a more open choke for 2nd barrel. Most ran Mod to IM first barrel, and full the 2nd. I liked IM first barrel, and my 2nd barrel to be the Hammer of God, as that bird is heading to the fence. .025/.040 was my choice.