Ive had some people tell me #10s will stop in the thighs and breast on a going away shot, and you dont want to try and pick out those... You cant even see the hole they make. Ive tried 1/4oz #10 tss 3/4oz #4 steel duplex but only at the patterning board, I was planning on using it for decoying ducks. You need speed with this stuff to get it to open up, but 1600 might be a little much. However be aware your cloud of 250 #10s are going to absolutely hamburger a pheasant sub 30 yards, even with cyl choke its not going to start opening up really until 30-40.
You might also look into #9.5, youll have 205ish pellets (similar to 1 1/4 lead 5).