I shoot 5.5 or so gr. of titegroup in mine with 200 gr bullets. I shot some black and 777, but the Remington is a light pistol and the recoil of a case full of 777 is very noticeable.
The Remington "58" does not get along well with real black powder as far as fouling is concerned, it fouls to a stop much sooner than a Colt type, but the advantage of the R&D conversion over the cap & ball cylinder is that the removal of the cylinder pin every time you reload gives you the chance to clean it off between loads.
One way to reduce the kick is to load 45 Schofield cases as they use about 6 or 7 less grains of black powder. The later R&D cylinders might accept them, but I had to open my rim recesses up with a 17/32 end mill to accept the slightly larger rim diameter. While I was at it, I milled viewing windows so I can see the rims and know which is the empty chamber more easily, but I think all the newer R&D cylinders already have the windows in them.