I'd agree on dropping the charge to about 80-90 gr.
I'm at my son's for Christmas and he told me this story:
He was using the .50 cal 295 gr. Powerbelt with 90-100 gr Pyrodex. After hunting one day he decided to see what kind of penetration they got. He shot into a 6" tree with the 90 gr, then tried 2 pellets, then tried 3 pellets.
One of the first two shots took a chunk off the side of the tree - nomeasurable penetration, since it tore off sideways. The 100 gr load penetrated the tree 4", but the 150 gr load only penetrated 2". It flattened out so much on impact that it couldn't continue good penetration. He also said the recoil was atrocious!
Admittedly, that was a tree - but many have testfied to the fact that a similar thing happens when a Powerbelt driven by charges over 100 gr hit a deer-sized game.
Back down on the charge, enjoy a much reduced recoil, and get better performance from your Powerbelts.