I've put together several, Uberti Colt Navies with both Kirst and R&D, a drop-in in an Uberti 1860, 7 Remingtons, Uberti and Pietta, Kirst 38 and R&D 45, and two SS Rugers. They all work well. The hottest loads I've fired in any were with 777 powder, which with a filled case of 45 Colt feels like a magnum, I shot that in the ROA but I would hesitate to use that in a thin walled Remington cylinder. The factory "Cowboy" loads such as Black Hills or Winchester are recommended by Kirst and R&D, don't go hotter.
The Navies, both Remington and Colt type require the use of Hollow Base bullets to open up to the 375 bore diameter, I use 148 gr HBWC from Remington or Speer, the Hornady bullet leads the bore. Unless the bore gets relined, I't's harder to find factory ammo in the Navies, as most factory wadcutters seem to use double-ended WC.