As others have said, I'd not worry about lack of power with any minie load. Their main drawback is a very poor trajectory. Standard minie loads do not much over 800fps at the muzzle and zeroed 3" high at 50 yards you'll be 6-8" low at 100. That places a premium on range estimation and hold over becomes pretty chancy past 75 yards.
By comparison, the same 70 grains behind a .50 caliber roundball will run about 1600 fps and with the same zero, 3" high at 50, will be dead on at 100 and only about 4" low at 125. That takes a lot of guess work out of your hunting since a center of rib cage hold will be a clean kill anywhere from the muzzle to 130 yards or so.
In a .58 I would definitely prefer a patched round ball over 100-120 grains for a velocity of 1300-1400 fps. That load would be at least as deadly as a minie and a lot easier to place well if your rifle will shoot it accurately, and it should, though it will probably require some modification of sights to get it zeroed.