Gandalf Vs. The Witch King

[Raynor]

There is a letter that indicates that the Witch-King was in some way enhanced for The Return of the King, but it's not clear whether Tolkien means narratively enhanced, or literally souped-up by Sauron.

To me, the later seems to be case

Originally Posted by Letter #210

The Witch-king, their leader, is more powerful in all ways than the others; 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.

Glorfindel and Ecthelion were virtually demigods.

As you have stated yourself, Glorfindel enhanced status appears after his death, as all reincarnated elves are, IIRC. That passage you quoted is irrelevant for Glorfindel at the time of his battle; the same goes for Echtelion. In a "mirror" argument, if he was greatly enhanced afterwards, his initial power, compared to his new one, is greatly reduced.