My buddy and I did some testing last week in Saskatchewan with our 28 ga on decoying snow geese. We shot our limit with 6 steel, 4 steel and 3 steel black cloud. I was surprised how well 6 steel killed snow geese. I've found a load with at least 120 pellets to be very effective if the pellets will penetrate. 6 were good to 30 yds, 4 to 35yds and the #3 were good to 45 yards.
I imagine a 1 1/8 oz load of 18cc pellets would kill birds way out there. Could definitely drop down payload and still be very effective I think.