Ballistics would be something like 140 grains at 850 fps through a 5.5" barrel. That's about 225 ft-lbs, or 0.53 lb*ft/sec momentum. I'm not sure what the ballistics for conicals are. Something like 200 gr at 800 fps?
The classic .38 SPL FBI load is 158 gr at 890 fps through a 4" barrel. 278 ft-lbs, 0.62 lb*ft/sec.
A standard pressure .38 would be 158 gr at 755 fps, 4" barrel again. 200 ft-lbs, 0.53 lb*ft/sec.
A .45 ACP will shoot a 230 gr bullet at 850 fps through a 5" auto barrel (equivalent to about a 3.5" revolver barrel), for 369 ft-lbs, 0.87 lb*ft/sec.
So energywise, a 1858 with round ball is somewhere between standard pressure and +P .38 SPL, from a shorter barrel. If comparing equal barrel lengths, it's probably about the same as standard .38 SPL. And it doesn't really stack up to a modern .45 at all.
Conicals may do better though, or even better would be a lead hollowpoint, similar to the old FBI load.