John,
I used to shot lots of decoyed snows/blues with lead 3" 1 1/4 oz. #6 lead 20 ga.in the "old days", it was deadly. My brother currently uses 3" Kent tungsten matrix #3's on geese with great results. You didn't say what type of gun you were using, but if it can take new, modern loads your 20 will do the job.
I have no personal experience with turkey, but two friends, both avid turkey hunters, use 20's and 6's with great results.
The problem you will encounter with steel 20 ga. loads is most have fairly low velocity and small payloads, with steel speed kills! You'll be ok on short range decoying ducks but I wouldn't expect too much from steel 20's on big birds or anything much over 35 yards.

Steve

"Every one must believe in something, I believe I'll go hunting today."