Surprised you found plated shot an improvement over good hardened (hi antimony) shot. Most of the plated shot I have tried worked just the same as soft shot. I think that was because the manufacturers used soft shot for the plating which is really not thick enough to add structure. Good quality hard shot does the best for me.
That's been my experience as well...I have a several decades old 25bag of original Winchester Lubaloy...Copper Plated #4 shot...and it's the real deal. It hasn't been made in decades and in spite of posting on a couple different hunting sites asking folks to look in garages, old barns, etc...I haven't come up with any and would think I'd died and gone to heaven if I could find a bag of Lubaloy #5s and #6s.
In the past couple years I've bought bags of Lawrence #5 and #6 copper plated, and bags of Ballistic Products #6 and #7 nickel plated...none of them are worth a $#@*...just pouring them in and out of metal coffee cans a few times knocks off the coating so pretty soon you look in the can and you see dozens of little shiny silver spots where the plating is nicked off, shining back up at you...will just save them for the skeet range or something.
When I patterned CP #6s, then NP#6s, to see if they'd tighten my long range turkey load of Magnum #6s, each of them averaged only an extra 3 pellets, with NP being better than CP.
The real sleeper was Ecotungsten/Niceshot...average # of Magnum #6s on a sheet of notebook paper at 40 yards was around 60, and Niceshot pumped in close to 80...significant jump.
Niceshot is expensive, but as a direct lead substitute, you can do all your load testing with the same size lead.
And no more shots than I take at a turkey (or goose) head, with a pound of Niceshot yielding about 10 shot charges per pound, a couple pounds will last me a few years.