After trying 4s, 5s, and 6 shot over the years; my hunting buddy and I have settle on shooting 7 1/2 shot at pheasant, wild or pen raised. What we have found is that 7 1/2 shot seems to penetrate better collecting less feather. I mostly hunt pheasant with an English 12 gauge box lock and my favorite load is B&Ps Competition One 1 oz. load of 7 1/2s. They patterns very well in my gun (choked IC & M) and kills pheasant very dead. Even on late season wild pheasant I've found 7 1/2s work very well. Late in the season when hunting wild birds I switch to either my Parker GH or a Belgian Guild gun both with tighter chokes ( at least in one barrel the Belgian gun is choked cylinder and full) but the shot size used is still 7 1/2.
We do hunt over very good pointing dogs and we don't take that many 45 plus yard. Even in that 40 to 50 yards range 7 1/2s seem to kill well.