In your example where you didn't get exit on the deer, again, more bullet, not more speed. You need more lead, better penetration than that ball. Round ball will kill, I know, but the sectional density is pathetic. That, and the momentum of a bigger bullet will give you total penetration. Also, a proper minie or Lee conical will give you better wounding being as it's big and pretty flat pointed. Round ball just don't sound like it's cuttin' the mustard for you and a 1:48 WILL shoot a conical. I doubt you could get a sabot to stabilize, but a 250 grain or so conical will.
If you are stuck on using round ball, you need to go to a .54 or .58. Like I say, things were simple back in the day. To kill bigger animals you just used more lead. It wasn't about velocity or more powder charge. Black powder isn't terribly good a producing velocity. It will push a ball only so fast. That's why the 8mm Lebel didn't come along until after Alfred Nobel was born.