If Sauron did know the origin of the Istari, it wouldn't have bothered him much, especially leading up to and during the war of the ring. He expected 100% to get the ring back, and in that scenario, all the Istari and white council together would be a small matter for Sauron to destroy. Even Gandalf says that if Sauron finds the ring, nobody can foresee the day when Saurons dominion would end, so powerful would he become. Which to me suggests that only the Valar themselves would be powerful enough to take on Sauron with the ring.
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