While Pedersoli lists quite low maximum loads - (someone advised they list 85gr. as maximum in my .58) that has to be a Lawyer's listing. In a .58, it's what I would call squib load. We are talking round balls here, not 600gr. slugs.

For me, that's a deer load. I'm with Dan - the larger the bore, the lower the pressure for ANY given load. I would test with good patch and proper ball size to the bore, right up to the normal 12 bore 2 3/4" heavy load of the 1890's - 7 drams.
I used this round ball load in a 7 1/2 pound 12 bore double, as it generated 1,550fps. We know fps and pressure are somewhat joined in BP shooting round balls. Thus a 12 bore producing 1,550fps is generating similar pressure to a 20 bore producing the same velocity.
The only real difference is recoil. The 600gr. ball is going to kick a LOT more than a 320gr. ball, when driven the same speed. The stock, has to be up to it. The pressure generated is actually quite low, even as muzzleloading rifles go.