If she is Playing 10C I would find a 10B team. The level of Play at 10 A/B is actually higher than most real12c teams. If she is dominating at 10B then moving up to 12's would probably be the better option.
I don't have a clue my daughter is only 10 so i didn't push real hard. I wonder if the heavier bats have thicker side walls = less pop? or is the extra weight generated with heavier endcaps? I haven't thoroughly looked into it.
I Had the best High school softball coach in our area tell me to never go -9 even through high school. She said stick with -10 Even for my power hitter.