I've only shot a few animals with round balls out of a .54 that I have. But, I've had fantastic penetration in each case. One mule deer I shot from above, off a cliff. The ball entered the top of his back and traveled clear down into his foreleg. Buck never took a step. A pronghorn I shot was facing me at about 6-70 yards. Ball entered the front of his chest and traveled the length of his body, breaking his left hind leg. Only went about 15-20 yards before dropping.
I've never shot an elk with a round ball, but wouldn't hesitate to do so, though I'd personally keep the range under 100 yards for elk, and would only take a broadside, or near broadside, shot. Kinda like archery hunting for elk, but with double the range.