Tom- I've seen guys posting using up to 175gr. in those rifles and getting regulation. I know it's over what the mfg'r lists as maximim, but note that also the big bores produce low pressure - your 80 to 120gr. loads are really 'soft' as 12 bore loads run. My own single shot rifle in 14 bore really likes from 140gr. to 165gr. for a hunting charge.

I would not be concerned with increasing the charge to whatever is needed. Even 175gr. of 2F GOEX will not produce over 15,000LUP - generally considered around max for round ball muzzleloading guns of caplock or flint ignition.