I have done some similar cautious experiments with 180 cast and 4198 powder in a Henry .357 rifle. Accuracy of a case full of 4198 is excellent for some reason, but velocity is under 1300 fps. I have to load at 1.6" oal. So a little duplex charge is the way to step up speed. Probably a dead end for me, but my loads and results have a lot of similarity with yours. Of course a normal charge of H110 or Lil gun will make plenty of speed without the hassle. Duplex loading gives us yet another opportunity to make a mistake at the loading bench