Sounds like one heck of a rifle you got there. It would be a lot of fun to shoot it. As for the roundball expansion, yes it will. Especially if you hit bone because you are pushing it with a lot of powder. What you might want to do instead of going to a smaller harder ball, get some round balls, the size that shoot the best in your rifle, and cast or have them casted with a higher BHN. They are still going to expand a little but not as much. You might also get some defelection with the roundball. I shot a doe this year with my .54 caliber and after smashing her rib behind the front shoulder, it then followed that rib up and took a perfect chunk out of her spine.

All in all it sounds like an exciting hunt and a great experience and test for your rifle. Good Luck