So Sauron is a maia. The maia were forbidden to use their full power. But there is nothing that restricts Sauron of using the full extent of the maia power. It was said that the maiar were forbidden, but if they are only but forbidden then could a maia access all of the power? And we are talking about a fallen maia so Sauron using his full power would be logical.
Yes, but that power was very limited compared to his former self. Sauron put most of his power into the ring when forging it, so when parted with it he is crippled...still an incredibly formidable foe, but not like he would be if he had the ring.
The reason the Istari(5 Maia sent to aid Middle Earth) didn't use their full power against Sauron was because they weren't allowed to. They were supposed to assists men, not save them.