Although I am also a fan of using coarser powders in bigger bore guns, I see no problem with using Fffg in a .58.
The joy of using Fffg is it is cleaner shooting, and in order to achiece whatever velocity, it doesn't take as much. I think the general rule of thumb is to decrease the Ffg charge by 10% for your Fffg charge. So, if you were using 100 grains of Ffg, you would only need 90 grains of Fffg.
To illustrate the situation even more, one time I chronographed my 8 bore rifle. I started out with the normal charge of 300 grains of Fg powder and the chronograph registered 1,425 fps. I then poured 300 grains of Ffg powder down it and registered 1,535 fps. Still having some caps left and a small quantity of Pyrodex Select Ffg, I gave that a try. Surprisingly enough the chrono said 1,604 fps after a significant hangfire. Just love that phony stuff, it always right there, eventually.
John