Isn't the point of nickel plating to make the shot slicker when passing through the bird/feathers and less about hardness? It's possible that I'm just a victim of marketing gimmicks but I thought that's what I had read.
I cannot answer that positively, but can say definitely that it improves patterns, same way copper plating does. And unless shown to be wrong I believe the plating imparts a bit of hardness that helps prevent shot from becoming quite as deformed upon setback. This bit of a harder shell is why some of us insist on true plated shot and not washed shot. The coating is just thicker when it's plated, thus imparting the hardness.
That, plus the possible increased penetration factor, would make it deadlier.
