Honestly, would been better if DMG have it's Manga effect where it gains 500atk instead of 300.
And it's more make sense aswell, because DMG suppose to Surpass her teacher, so having the same attack when her teacher is in the GY is Thematic.
The MC's Ace card would be "Dark Magician Girl, since it's her only "duel spirit" it would make sense, maybe you can invent fusion, ritual or effect monster cards that use Dark Magician Girl as a core.