I would suggest changing it to say something like mastered a small fragment of AM instead of the whole thing, you can have her possess the same level of power even with that, and it doesn't lessen either Sakura or Angra in any significant way.
I might also add that one does not need to master second magic to travel between worlds, that is to say that if she was a master of the second she would be Zel level automaticaly. She could just as easily use a supercharged version of shadow transport to move between worlds.
Just my thoughts, do with them what you will.