[Ibrîniðilpathânezel]

Gandalf's greater overt ability after his return from death was directly related to Eru changing the scope of his mission and his ability. I'm out of town again and don't have the Letters with me, but Tolkien said that Eru took his sacrifice as an opportunity to enlarge upon the Valar's original plans at the moment of their failure, and sent him back in an enhanced state to answer the greater challenges he would face. So in a way, his power had been increased because the situation had become more dire with the overall failure of Project Istari, but I don't know that one could say that Gandalf was allowed to use more or less power depending on the situation right from the beginning of his mission.

Whether or not he was a great "wizard" is not the same as being a powerful "magic user," in Tolkien's world. The term "wizard" is, in Middle-earth, an inadequate translation of "Istar," a term which actually refers to wisdom, not power (which, etymologically, is the actual root of wizard, "wise"). In that respect, I believe Gandalf acquitted himself quite well.