Lee - Seems as though my rifles are just the opposite of yours. I haven't had a rifle yet that shot good with loads over 100 grs.
My Genesis likes 100 gr of Pyro P but over that it starts getting erratic. The little Knight that started this post is shooting 90 gr of Pyro RS under MOA. And all my sidelocks prefer Pyro P or FFg or FFFg in the 80 - 90 gr charge. This is with everything from PRBs to 250 - 320 gr lead conicals and 240 to 300 gr saboted bullets.
I don't dispute your claims bu I guess I am lucky in that I don't have to dump huge amounts of powder down the tube. And its easier on the shoulder too.
Now too, I may be wrong, but in a 28" or shorter barrel, it is my opinion that, because of the burning characteristics of BP and its substitutes, anything over 100 grs is being burned after the bullet leaves the barrel. Other opinions please?