Whatever you decide to do I do not think you will regret going to a larger bore. I hardly shoot anything smaller than .54 now. Thats mostly because of the calibers of the guns I own, but I think I will be grateful for the size when I go hunting with one of them. .54 or .58 is good for anything up to elk. There is also just a fun factor of slinging a larger ball. Yes you go through more powder and lead, but I have noticed (at least in my experience) all forms of blackpowder shooting is significantly lower than that required to feed a habit of using unmentionables. If the rifle can be bored to .58 I think you will certainly be happy. However, if the biggest you can go is .54, I think you will still really notice the difference, and be happy with the performance. If you can do .58 I would do that and be very happy. But if the largest you can go is .54, I think you will still be happy.