I've owned one and spent time with three .54 cals. One had a 1:60" twist. The others were 1:48".
They all shot roundballs quite well.
I now, begrudgingly, have a .50 cal with a 1:48" twist. (I want another .54!) It shoots roundballs great. So well, that the conicals that I bought in anticipation of being disappointed by RB performance have not been opened.
I will say that I have found the 1:48" twists to do better with lighter powder charges than most people *want* to run. The sweet spot in the .50 and my first .54 was 70-73 gr Goex FFg (by volume). They still shoot "minute of deer" or better up to 90 gr; but best groups were at lower velocity. On the other hand, the 60" twist shot great, all the way up to 110 gr FFg.
Performance on game has been great. Every recovered ball showed that it impacted with a stable spin, and mushroomed and continued penetrating while remaining stable (until or unless it hit bone).
I saw my first RB kill and recovery in 1998. I was sold when I saw that mushroomed ball. I haven't seen another that disappointed me since then.
My Optimum target shot-string is more critical than my hinting load. The TC manual and others, list these loads but range-time, is always in order, for your confirmation.
My current hunting muzzleloader, the .50 cal, must be fired at least once and fouled before it can be trusted. The first shot always goes way high - like 8-9 inches - and usually has some windage as well. But after that, it'll shoot dead-nuts, at about 1-3/4" at 100 yd if the shooter does their part. On a bad shooter day, it is still a 4-minute or better rifle. Good enough for the intended ranges and game.