The Limitations of the Istari

[Nikkolas]

I have read the part of Unfinished Tales that concerns The Istari (at least the parts about the council that took place before they ventured to Middle-earth) and it covers how their "might" is hampered by the flesh they must wear while performing their Task.

However everyone I've seen online makes it out like these were self-imposed restrictions. For example, Gandalf simply chose not to show his true power save for when he had need of it ie. when he fought Durin's Bane, a fellow Maia.

Where this really concerns me is in the case of Saruman. Many people speculate whether or not he could command the One Ring. He'd need to have enough innate Maia power to combat most of Sauron's power in the ring after all. However if he was truly limited by simply being in his "human" form, he could never have achieved this anyway.

But if the limits were just guidelines that could be thrown off at will, then surely Saruman, having been corrupted long ago, would wield his full power without hesitation.

Are there any official quotes one way or the other on whether the true strength of the Istari is held in check only by their will? Thoughts and opinions are of course very much welcome too!