I know what we use now is far more powerful.
Over the roundball out of a percussion pistol, yes but blackpowder cartridges, not really. It's still tough to improve upon the original .45Colt blackpowder load. A ~250gr bullet at 900fps is still mighty potent. The issue is really smokeless powder's pressure and pressure curve. Which is why we like to keep our blackpowder designs like the open top Colt's to about 700-750fps with smokeless powder.
There's a picture of a Lemat pinfire conversion on page 29 of Dennis Adler's book on metallic cartridge conversions. Worthy of note is that the French Lefaucheaux pinfire revolvers also saw extensive use during the war. That at the time, they were considered superior to domestic percussion guns and the French patent for bored-through chambers preceded the Rollin White patent by a full year.