I have a .45 longrifle, similar to what you descibe, with a 42" barrel and 1:66 twist. I've killed six deer with it, using 55 grns of fffg and patched round balls. All but one shot were passthroughs (including penetration through shoulder blades, front leg bones, and spine), and all deer dropped on the spot or ran <80 yds (across an open field). Shot distances have been between 15 and 100 yds. The one that didn't pass through "apple-cored" its way, lengthwise, through the center of the spine, and accounted for one of the deer dropping in its tracks. I never wounded one, so I can't vouch for what a .45 roundball may do on a gut shot vs. a .54, but I would suspect similar results.
If/when I were to build or buy another longrifle, I think I'd stick with a .45. IMO, the pros are it having almost no recoil, having a very flat trajectory, being perfectly adequate for deer, and being inexpensive to shoot with low powder loads and cheaper balls. The only con to me is that I may want something larger if I were to go after bigger game.