TOF - you may have a point about some barrels, the slower powder giving higher speeds - Forsyth thought so as well, but of course, he was talking about large bores as well as the Minnie-firing rifles of the day.
In my own 14 bore, 3 drams of 3f gives higher speed than does 3 drams of 2f and equal to 4 drams of the slower stuff, however I cannot increase the 3f very much more before gas leaking problems surface, however with 2f, no such sealing problems 'ever' happen - to the limit of this human's recoil endurance. That part I find interesting.
The old rules of thumb still preside, I think - the longer the barrel, the slower the powder - yet the Musketoon does very well at 1,300fps with a mere 2 3/4 drams(75gr.) 2F in it's stubby 24" bl. Today, GOEX appears, in this gun at least, to give 200fps higher velocity than the GOX of Lyman's book time and is way ahead of the C&H we used back in the 70's.