I haven't used my 54 for a few years. Back when I used it I used 430 gr TC maxi hunters and Buffalo Maxi-ball and Ball-et over a 110gr of FFG. Both worked very well.
Most guns shoot the Buffalo Ball-et very well although they are pricey compared to many.
I say lets all remove the warning labels and let nature take its course.
Cabelas 210 grain conical shaped bullet. 125 grains of powder has worked well fo us and we spent a ton of time and $$ in our quest.
Anythig heavier and the speed of the projectile is so slow you can see the bullet when you are sighting in.
I've been wondering about the .54 vs the .50 myself. I was concerned about availability of decent bullets for the .54. Wasn't sure if anything but round ball was available. Good to know.