After almost 5 years I admit it: Seraphine is indeed better as a mage
I played a few games building full AP and now I simply cannot go back to the enchanter playstyle. It feels so weird not dealing any damage once you get used to it. Also, you can only be efficient every 13 seconds on late game, since your other skills don't do much without damage.
My problem with AP Sera was because she was a hyper-late game champ, with little base damage and a bad 1v1. Also, I always found it a bit troll as a support because her kit needed a LOT of gold to be efficient.
With that said, I still find her more suitable as a supportive mage than a hypercarry who gets all the kills.