Quasit is way too good for +0. Forget about 9th level spells. You're talking about exactly level 20 and no other levels. You can't balance for only one level.

I'd be with you if it were a humanoid, but this is an outsider. 3 HD are barely a drawback at all. Especially since you can go into Ur-Priest or Ardent or Sublime Chord and have full 9ths anyway, not even behind schedule.