Will 7 1/2s completely penetrate a pheasant at, say, 35 yards?
In general, no they won't.
For game farm birds, you should be OK with 7 1/2 shot.
For wild birds, I use 5's for the first 2 weeks, then switch to 4's for the remainder of the season. Even 4's don't generally pass through. On the other hand, #4 Hevi-Shot will consistently pass through Mallards at 45yds.
I will also carry shell's loaded with #2 lead. Many times my right barrel has 4's in it and the left has 2's. Since I no longer have a bird dog, I need to make sure they are dead when they hit the ground, and 2's will do that.#2 lead typically perforates roosters completely.
