The problem these days with "copper plated" shot is that the copper is mostly just washed on. It can make the pellets easier to find when you're cleaning the birds. Other than that, the "plating" doesn't add much.
If your hit on a pheasant isn't fatal, then you'd better hope that the bird has a broken wing and a broken leg.
Do you have any evidence that this is the case with BOSS copper plated bismuth? They make their own shot and I have seen no information about whether it is truly plated, or washed.
There is true nickel plated lead shot available, and I load it in a specialty 3", 3/4 oz. dove load for my .410s. It kills like blue lightning, as compared to unplated equivalent lead shot. I can really tell a difference in mortality vs cripples.
