When I first started hunting elk with a muzzleloader, I thought it was all about power. I won't even tell you what powder load I was using the first few years :redface: But gradually over the years I realized that shot placement is most important, and even with non-magnum loads, penetration with a round ball is never an issue.
With the .54, I settled on 110 grains goex ffg, plenty of power, but most importantly, still accurate. I took a bull at about 60 yards right between the eyes with that load. As for penetration, one time I took a cow quartering away from me that was hit just in front of the rear leg and the ball ended up in her chest. I figured it was close to 30" of travel with a dead soft pure lead roundball. I also took a cow broadside using another rifle with a 26" .54 barrel and 100 grains ffg. The ball destroyed the lungs, broke a rib on the far side, then bounced around in her innerds for a while. Like I said, with a roundball, penetration is not an issue.
Like previously mentioned 80-90 grains is fine for a .54 roundball used on elk. The only reason you may want to power it up a little is the consideration of trajectory. The .54 can be a pretty flat shooting round when enough power is applied, so a flatter trajectory may be desireable for those quick shots, without the need to calculate up hill, down hill, range, and wind deviations. Not as much anyway.
All my elk kills have been under 60 anyway, so I don't worry much about trajectory. Anyway, I swithced to a .58 for elk.
Good luck. Bill