As far as IMR 7828 being THE best powder for your gun and bullet combo I cannot honestly say. According to my old Nosler manual you've hit the nail on the head. It's their best preformer from what they list. I would think that the Partition would be a great choice also. I was curious about the Grand Slams you mentioned as not doing well. In what reguards did they faulter.
Good Luck on your development and your hunt.
Mike / Tx