I've used nothing but 7.5 for all my pheasant hunting with my 28 Gauges, behind pointing dogs and never had a problem putting them down.
I think anything over that is overkill, limits the amount of shot and will produce holes in the pattern.
Unless you've patterned your gun with a specific shot size, weight and choke, your just having luck if you hit anything with regularity.
If only shoot at pointed birds 28g should do fine
But if you are spending money and traveling 1000 miles
Hunting wild birds
You need 12g and the right shell and shot
My .02