If you will look at the pellet energy chart in the Lyman Shotshell Handbook, you will see that the distance from the muzzle to the bird is all important. Joe, the reason your #9 shot was killing ducks so well is that they were within 20 yds. The longest target to hit at Skeet is only 22 yds. which is why #9 shot is used in that game when the winds are not too bad. The same would be true of small shot hitting a turkey. As long as the bird is at short range, all those little pellets combined carry much more energy than a few large pellets. They do kill by shock, and not usually by breaking the neck, or penetrating the spine, or head.
For readers who don't have the Lyman Handbook, #9 shot, leaving the barrel at 1200 fps. has 2.40 ft.lbs. of energy at the muzzle, and 1.12 ft. lbs of energy per pellet at 20 yds. Assume 1 lbs per pellet in ft. lbs energy for our purposes. If you can hit that bird within 20 yds with 30 pellets, you are hitting it with 30 ft. lbs. of energy, and that should stop it. Whether its just stunned, or is actually killed you will have to determine by examining the bird. You don't want a large bird like a turkey reviving while you are carrying it out of the field, or in your car!
Now consider the energy of #4 shot( .13" diameter) fired at 1200 fps. It starts at the muzzle with 10.36 ft. lbs. of energy, and at 20 yds has 6.29 ft lbs. It would require only 5 pellets to hit that turkey to produce the same energy effect on the bird as 30 pellets of #9 shot!
Or #5 Shot( .12" Diameter ) at 1200 fps, it has 8.22 Ft. lbs. of energy for each pellet, and at 20 yds, still retains 4.78 ft lbs. of energy. About 7 pellets will deliver that 30 ft. lbs. of energy on a turkey.
Or #6 Shot(.11") diameter) at 1200 fps, It has 6.20 ft.lbs. of energy at the muzzle and at 20 yds., retains 3.49 ft. lbs. of energy. About 9 pellets will produce that same 30 ft. lbs. of energy on that turkey.
From my own experience, you can definitely tell the difference when a bird is hit with different shot size in the air. One of the first times I went pheasant hunting, a bird got up in front of me in standing corn. I fired and hit it with an 1 1/4 of #6 shot, but the two guys I was with, to my left, also shot the bird at about the same time. The bird was maybe 15 yards in front my barrel, about 22 yds for the man next to me, and 30 yds for the third man. #2 man hit the bird with a similar load of #5 shot, and #3 man hit it with a similar load of #4 shot. The three loads of shot hit the bird before it began to fall to earth. My shot caused feathers to fly, and the bird's wings to begin to fold. The #5s caused it to flip over, and more feathers to fly. And the #4s smacked it like a tennis racket hitting a ball and knocked it out of the air. It was pretty badly torn up. I remember letting one of the other men take the bird, and saying something to them about my zone of fire. They had never hunted with me, I was carrying my father's single shot shotgun, and since they had never seen me shoot, they were a little quick, lets say, to " back me up ".
MY recommendation remains to use a larger sized shot when hunting turkey. #7 shot, ( .10" diameter) is only occasionally sold here in the U.S. but appears mostly in international trap loads delivers 4 ft pounds of energy at the muzzle using the 1200 fps loading, and 2 ft lbs. at 20 yds per pellet. That would give you twice the pellet energy as the #9 shot, but a third of the energy delivered by the #4 shot, at 20 yds.
Those layers of turkey feathers can take a lot of energy and bounce off the shot- its been seen. A friend of mine worked up loads for his 20 ga. smoothbore fowler with the large shot, and the small #6 shot. He decided on the #6 shot, because his gun was putting so many pellets on the head and neck target he was using at 25 yds. He killed his turkey with it, called in to about 20 yds. I think we counted 12 hits on the head and neck. I don't think he was shooting enough powder to reach that 1200 fps. load, but even at 3 ft lbs of energy at that range, he hit the head and neck with 36 lbs of energy, and at least one of those pellets broke the neck.
Best Wishes on a successful hunt.