Hello Dean. I went through the same process, a few years ago, looking for more reliable ignition in my muzzleloader. While not the same as a PRB, or with true BP, here's what I experienced.....
I shoot a .54 remington inline with hornady 425 grain great plains bullets. I used 135 grains, by volume, of pyrodex RS select (FFg). When I switched to pyrodex P powder (FFFg), I started out with a reduced load, shooting over a chronograph.
The RS load was 1650 fps. I worked up the P load until I hit 1650 fps. It was 110 grains by volume (87 grains by weight). Then I shot for accuracy.
Group size was the same, and POI was the same. The gun showed no difference in regards to accuracy or POI. I was really pleased!
The upside of the finer powder, was not only using less powder, but the difference in ignition was very noticeable....much quicker. The gun had a different report sound, more of a crack, than ka-boom. And less smoke hanging in the air.
Since the switch, I've killed several bull elk, cow elk, and a few deer. The gun and load combination has performed flawlessly, under all weather conditions found here in Idaho.
It has my total confidence. That's more than I can say for the old RS/FFg load.
Andy3
