I don't see why Sauron would even be too concerned with Saruman being a Maia. After all, bound in the body he was, he certainly could not oppose Sauron in a test of whatever strengths the Maiar had. My theory is that he knew what Saruman was, what he represented, and what he was capable of, and deemed him not only not a threat to his power, but in fact a useful tool in his plans.
I'm really not sure Sauron would have much respect at all for the Istari and their task, if indeed he did know about it. Bound as they were, they could not directly oppose Sauron, and were in fact susceptible to corruption. I think that Sauron would have been amused by the feeble plans of the Valar, and would have not have feared the Istari, but rather adjusted his own schemes to make the best use of them... which is perhaps what he did.
Which brings me to another point. Did "human wizards" actually exist in Tolkien's universe? I don't recall any humans that had Maia-like powers in Tolkien's works, and it doesn't seem to me that any man would be able to perform the feats of an Istari. And of course, if there are in fact no "human wizards", this would be another big hint to Sauron that Saruman et al. aren't really human.
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