[Raynor]

I don't think that these arguments:

Originally Posted by CSteefel

Clearly, the Witch King is feeling as if he can stand up to Gandalf, although it is not clear that understands completely that Gandalf's own power has been substantially enhanced.

Otherwise, the Witch King's breaking of the Gates of the City might be seen as similar to Gandalf's sundering of the Bridge at Khazad-dûm with his staff. This in fact succeeded in sending the Balrog into the depths (even if it did not kill him), and only by a combination of skill with his whip (and perhaps a bit of luck) was he able to take Gandalf with him.

would justify this conclusion:

So I'd say the Witch King, at least with his added power, would be quite a match for the Balrog, if it came to that.

The breaking of the bridge was a rather unique event that helped in the end the defeat of the balrog, with little if any chance of its necessity or conditions appearing again. While confidence is definitely an important factor in a battle, it would not suffice in overcoming a balrog, so the WK's belief he can win over Gandalf does not say much about how he would do against the balrog. In some sort of a Condorcet's paradox, we may even find that Gandalf can defeat the balrog, WK can defeat Gandalf (presumably) and the balrog can defeat the WK. Although I think that the WK would generally lose against the balrog, I guess in the end we are talking probabilities of defeat/winning, so we can imagine that if they fought for a large enough number of times in rather similar conditions, at least in one battle the WK would win by chance.