Hey Sturgell, I have seen Mike Venturino do that same thing with 45 Colt. Mike used gas checks for his wads.
In his tests the lower powder charges seemed to pattern a lot better.
I wonder if your data taken from the Speer shot capsules is a touch warm because your wads are pressed down over the powder... with the shot capsules there is a lot of room and they aren't seated as deep.
I know seating depth can raise pressures quite a bit. Something to experiment with...