I don't see where you'd need a lot of "gear" in going from .50 to .54, just a different ball size. If you're casting your own balls, then yes you'd need another mold and a cleaning jag but that's a heck of a lot cheaper than a new barrel.
Perhaps T/C did build a .54 with a 15/16" barrel but I sure never heard of one. I think any 15/16's .54 caliber on a T/C would be a replacement barrel not of T/C's making.