Daryl
With all respects, but I have to let you know you are wrong in your assumption. I had a .58 caliber Hawken 38 years ago, with an old Green River barrel in it. It was the first muzzleloader I ever made I bought a REAL 440 gr mold. (which I still have) I killed deer and I also used it to kill cattle on the ranch back home from time to time (sick ones and the bulls when it was time to "move them off".
I found that the 440 gr REAL made from water dropped WW would penitrate extremely deep, but would veer off in some cases, and not go where I wanted it to go in the cattle. The balls went straight.
I cast some REALS out of pure lead and I thought (as you do) that they would do better than the balls.
You are also 100% correct in your idea of how a hard WW ball needs to be used in a 58. My old mold is a .565" and I used denim as a patch.
The surprise was that using WW in a .565" ball penetrated at least 2X deeper than the REAL cast from pure lead. WAY deeper and WAY better!
I know if seems unlikely, but try it.
A few years ago I killed a nice big moose with my 62 cal (.600" ball) American Jaeger rifle. I used a WW ball (320 gr) and it went through BOTH upper leg bones and exited the moose and I heard it hit some tree braches on the other side. I have killed 4 moose in my life and 3 of them seemed somewhat unimpressed with my bullets. All died, and pretty fast, but I was impressed how even the 375H&H didn't seem to "hurt" them all that much.
I killed 1 with one 375 H&H bullet (300 gr Sierra) one with 2 375H&H bullets (285 gr Speer Grand Slams) and one with a 348 Winchester (200 gr Hornady) None of them even staggered when I shot.

But the one I killed with my 62 flintlock dropped on his chin as soon as the gun fired. With just a hard ball, and it went clear through.
So, don't think WW balls won't penitrate. They will.
I don't think they will go as deep as an 8 bore bullet, but I can promise you from my own experience, in 58 cal, the WW balls did LOTS better then the pure lead bullets.

Steve