[obloquy]

It's right in the book dude. The quote was provided on the last page.

I'm aware of what's in the book; perhaps more aware than you'd like me to be. I do not contest the fact that Tolkien wrote this, I contest how it is being interpreted. The letter in question is a point-by-point critique of the unproduced M.G. Zimmerman film treatment of LotR. Tolkien explains not only the flaws in Zimmerman's revisions, but also the intended effect of his writing choices. The letter is not in answer to questions about the nature of the Witch-King's power at different points in the story; rather, it is an exploration of Tolkien's literary intentions. I'll reproduce the paragraph in full here, since its previous quotation in this thread deprived it of context:

9. Leaving the inn at night and running off into the dark is an impossible solution of the difficulties of presentation here (which I can see). It is the last thing that Aragorn would have done. It is based on a misconception of the Black Riders throughout, which I beg Z to reconsider. Their peril is almost entirely due to the unreasoning fear which they inspire (like ghosts). They have no great physical power against the fearless; but what they have, and the fear that they inspire, is enormously increased in darkness. The Witch-King, their leader, is more powerful in all ways than the others [though note that he is nevertheless not excluded from the above explication that the Nazgul rely on fear rather than any true power. -obloquy]; but he must not yet be raised to the stature of Vol. III. There, put in command by Sauron, he is given an added demonic force. But even in the Battle of the Pelennor, the darkness had only just broken. See III 114.

It is possible that Tolkien intended to convey that Sauron had enhanced the Witch-King in some way. However, it seems more likely to me, given the context of the excerpt and the manner of the letter, that Tolkien is describing the literary effect of Sauron's decision to put him in command of the assault on the Pelennor. We, the reader or viewer, see in the Witch-King in III "an added demonic force" that is due to his portrayal at first as a scary wraith, and later as a military captain. In the last sentence Tolkien still qualifies the Witch-King's appearance at the Pelennor by reminding us that the darkness, under which the Nazgul seem more terrible, had only recently passed.

Also notice how Tolkien says "he must not yet be raised to the stature of Vol. III." By whom must he not yet be raised to that stature? It sounds like he's telling Mr. Zimmerman that he must not yet raise him to that stature in his film, implying that the "added demonic force" (or increase of stature) of Vol. III is given by the author, not by Sauron. The command of the army is given by Sauron, which results in an impression of increased stature on the reader.

Whether you agree with my analysis or not, it is impossible for you to claim that Tolkien's intention was, without a doubt, that Sauron had pumped some extra bad-guy juice into his pet.

That is only your opinion in the end

It is not only my opinion if there is no evidence to support the contrary.

and the scene in the book where Gandalf met the Witch-King at the gates of Minas Tirith was most definitely not a confrontation where one combatant was depicted with any sort of strength over the other.

Again, I'll reproduce the whole segment so we don't forget the details:

In rode the Lord of the Nazgul. A great black shape against the fires beyond he loomed up, grown to a vast menace of despair. In rode the Lord of the Nazgul, under the archway that no enemy ever yet had passed, and all fled before his face.

All save one. There waiting, silent and still in the space before the Gate, sat Gandalf upon Shadowfax: Shadowfax who alone among the free horses of the earth endured the terror, unmoving, steadfast as a graven image in Rath Dinen.

'You cannot enter here,' said Gandalf, and the huge shadow halted. 'Go back to the abyss prepared for you! Go back! Fall into the nothingness that awaits you and your Master. Go!'

The Black Rider flung back his hood, and behold! he had a kingly crown; and yet upon no head visible was it set. The red fires shone between it and the mantled shoulders vast and dark. From a mouth unseen there came a deadly laughter.

'Old fool!' he said. 'Old fool! This is my hour. Do you not know Death when you see it? Die now and curse in vain!' And with that he lifted high his sword and flames ran down the blade.

Gandalf did not move.

Chilling to the spine, gorgeous and one of the best portions of the whole book. Gandalf stands in the way of the Witch-King, untrembling, unhesitating, and tells him that he cannot enter the city. The Witch-King obeys. The Witch-King confronted Gandalf by accident, not because he was planning to fight him. Gandalf was waiting for him, and prevents his advance. The Witch-King mouths off, but his words are empty and his threat does not even get Gandalf to move. The Balrog at least prompted Gandalf to prepare for combat! Gandalf may still be required not to reveal his true power, but he defeated the Balrog while observing that limitation, and he is here, at the gates, in front of the Witch-King, obviously unafraid. Granted, the Witch-King does not appear afraid either, but there's no reason to think that he had any idea what Gandalf really was, and he is notoriously overconfident, having misinterpreted Glorfindel's prophecy.

The tension of the Battle of the Pelennor Fields is due to the uncertainty as to whether Minas Tirith can hold out against the siege. It has nothing to do with which leader is personally more powerful.