Larry Pletcher has shown this to be true, at least with respect to the speed of ignition. If you live in a dry climate, the finer grained FFFFg is certainly the priming powder of choice because there's more surface area with it. However, in the often humid Hudson Valley where I live, FFFg works a bit better since there's less surface area to turn into "mud" in the pan, even if rigorously wiped between shots.