RE: Traditional & tight groups??
I also would recommend the .54 caliber. Whether I would suggest shooting 150 yards with open sights, all I can say is it will take a lot of practice. The Lyman Great Plains Hunter should shoot the Lyman Maxi Ball real well. The Maxi Ball will take care of any animal you want to confront.
When I hear people worry about the trajectory of a .54 VS a .50 caliber, to me it is the same difference as when I shoot sabots or conicals. Sure the conical has a more rainbow trajectory, but you sight the rifle in for that, and you learn the different drops so what's the difference? It's all a matter of practice.
The .54 caliber will carry more lead. More lead means more energy down range. While the .54 caliber shooting conicalstrajectory might be a little more rainbow then a .50 caliber shooting conicals there is not that much difference. And when the bigger conical hits, I think the effect will be more then appreciated.