A .530 ball weighs about 230 grains, so a 300 grain bullet isn't too much of an increase.
I recently acquired a vintage CVA Missouri rifle 54 caliber.
Haven't spent a lot of time with it yet but so far, the 530 ball and 70 or so grains [V] of 3f Goex seemed to be a good load.
I'm just not a fan of modern saboted bullets in a traditional rifle. I'm just old fashioned, I guess.