Best why to tell what shell is best for your gun is to hit the patterning board. But as someone mentioned, patterning does not give you the full 3D effect and shot stringing can be a big factor.
To kill waterfowl or anything for that matter with birdshot, you need penetration and density. Penetration being the pellets having enough energy to pentrate into the vitals and density being, enough pellets on target to ensure enough hits to the vitals.
More shot may not necessarily give you the penetration and density needed.
Everyone likes the real fast stuff and it is good, but a 1200-1300 fps load still has enough energy to ensure enough penetration to kill, at normal shooting ranges, ie. 40 yards or less.
I have guided for waterfowl for 20 years and I have yet to see someone that can consistently wingshoot past 40 yards.