I don't do turkey.
Used the 20 for ducks when lead in the 12 was first banned. As long as I didn't try to stretch things too much past 45 yards, shooting 1-1/4 ounces of nickel plated #4s that patterened quite well, the gun did the job far better than I had anticipated. It was way better than the 12 with the steel shells of those times. As soon as lead was banned in the 20, I went to a 10.
With the newer materials like tungsten matrix and other as-heavy or heavier than lead alternatives, I would think the 20 would be okay again, but I have stuck with the 12. I have a friend who likes to shoot the 20 at ducks, and last Fall even he admitted that his kill ratio dropped noticably with the 20, even limiting the range he tried things and over decoys. He does shoot steel in the 20 and the 12, and I don't. He is thinking of trying tungsten in the 20.
What could have happened... did.