In hunting Pheasant for over 30 yrs in both Iowa and SD, trying everything from combo shot,5-7.5 1/3-8, stright 6`s,5`s etc, finally setteled on 1/1-4 oz 4`s about 20yrs ago and never looked back. Used to load 800x but changed that to 32grns of Longshot at an advertized 1440fps. Very few runners...we also have found that concentration on the birds head when flushing or flying, places most of the shot string there, instead of the backend when you center the rooster. If you don`t hit him, you can`t kill em. We also found with 4`s it does not take many hits to kill cleanly...lots of penitration and energy with that shot size.
I shoot an older 101 O/U with fixed I/C and Mod chokes. I hunt over a dog, so many shots are close, which is fine with me. I will not shoot a bird over tall grass way out there, cause if I don`t kill him, chances are I will not find him, dog or not. I will choose my shots, cuts down on cripples. YMMV.