Well, you're stretching the extent of our speculation a bit, but it's possible.
I think it's more likely that when an Ainu was designated either Vala or Maia, its spiritual stature played a part in determining to which class it would belong. When I said this:
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it is also possible that some of the Maiar, and especially unaffiliated ëalar or Ainur, could have had spiritual power to rival some of the Valar.
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...I mostly meant those unaffiliated
ëalar, and specifically Ungoliantë, because of the potential she demonstrates in overwhelming Melkor. Arien also exhibits impressive power. How Sauron compares is anyone's guess. I think there probably were some Maiar that were greater than Sauron, but I doubt there were many.