Ëalar and Incarnation.

[obloquy]

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:

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.

...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.