A few things. The feat that Holy Mami scales from is not only done passively, but while she is focusing most of her power into an actual attack. The calc itself is a very, very big lowball compared to her actual AP.
As for stats, Mami has access to the Doppel transformation, which is stronger than the regular witch transformation, which is immensely above her base (For example, Sayaka is 7-C and her witch is Low 7-B, the witch of 6-C Madoka is 5-C, the witch of 5-C Madoka is 5-B etc), and she can access it when she wants.
Also, her ribbons, forcefields and attack reflection are even higher.
Basically:
Mami <<<<< Witch Florence < Doppel Florence < Mami's ribbons <<<<<< Florence's ribbons < Doppel ribbons
Her healing is actually pretty solid, magical girls are stated by Kyubey to be able to heal any kind of wound (including stuff that normally kill people like losing all your blood, being decapitated etc) provided that their Soul Gem doesn't break (it's the gem on Mami's head).
Her type 2 also works similarly (It's even better than the healing, magical girls can survive as a Soul Gem only)
Her power nullification actually works kinda like Silence in Final Fantasy, except that it comes with a ribbon trap to prevent movement as well (the ribbons are invisible). For reference, it has successfully nullified the abilities of witches similar to
H.N. Elly, as well as people like
Homura Akemi,
Kyoko Sakura, and a clone of Mami herself.
Absolute Territory, ribbon shields and Gun of Aegis can block/reflect piercing attacks.
Mami doesn't have power drain herself, she has a unconventional resistance (basically if you steal her powers, they either go away from the thief, or posses them)
