of Gandalf the White "He is still under the obligation of concealing his power and of teaching rather than forcing or dominating wills, but where the physical powers of the Enemy are too great for the goodwill of the opponents to be effective he can act in emergency as an 'angel'-no more violently than the release of St Peter from prison. He seldom does so, operating rather through others, but in one or two cases in the War (in Vol III) he does reveal a sudden power: he twice resuces Faramir. He alone is left to forbid the entrance of the Nazgul to Minas Tirith, when the City has been overthrown....."

I cannot currently find the source of the following but if I can be taken at my word, in sources outside the stories, I am 99%+ sure that there is text stating Olorin prior to taking on the Gandalf incarnation is noted as being fearful of Sauron in part at least because Sauron was much more powerful than himself, and hesitated in accepting the Istari mission. Similarly elsewhere, when discussing the Maia and their power there are words to the effect of "greater such as Sauron or lesser such as the Balrogs"


Sauron and the One Ring, Letter 131. "While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power was existed and was in 'rapport' with himself; he was NOT 'DIMINISHED'. Unless some other seized it and became possesed of it. If that happended, the new possesor could (if sufficiently strong and heroic by nature) challenge Sauron, become master of all that he had learned or done since the making of the One Ring, and so overthrow him and usurp his place." (my capitalisation and emphasis).


So as I read it he was not diminished unless another possesed the One Ring and mastered it.