Even from Sauron's point of view Sauron should not be making that evaluation - at least not about the ring in particular. While for Sauron the ring is particularly powerful and not-values-changing he knows that for Gandalf it is, in fact, corrupting and also that Gandalf doesn't get anywhere near the same power from the ring.
I think that part of Sauron's character is that he can't understand why Gandalf views the ring as corrupting. Maybe he should be able to figure this out, but I was describing how Sauron might think, not how he should think.
As for whether Gandalf would get power from the ring — It's true that the ring wouldn't give Gandalf as much power as it would give Sauron. But it would still give Gandalf a lot of power.
In some other essay, Tolkien speculates about who would win in a one-on-one fight between Gandalf-with-the-ring and Sauron-without. Unfortunately, ...
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