My experience with 3f has been that you lose the expected velocity advantage when you move to the heavier bullets...at least in my guns over my chrono. My hypothesis is that the bullet takes long enough to get out of the barrel that you burn all the powder anyways.
Of course, all the barrels I have tested are 26in. I have not tested my brush guns as I do not feel the small velocity difference makes it worth screwing with a second powder (I often carry 2 guns afield) when it's a short range proposition anyways.
Breech plug design improvements have certainly made the newer designs more user friendly.