You're quite correct - we have absolutely no cases whatsoever of the Ring conferring anything but the most basic of power on anyone aside from Sauron.
Of the actual Ring-bearers we have:
- Sauron himself - well, you know...
- Isildur - wore it for a few years, invisibility and possessiveness.
- Deagol - never actually wore it, was killed shortly after he found it.
- Smeagol - wore it for hundreds of years, longevity, invisibility and possessiveness, small and mean evils.
- Bilbo - wore it for many decades, longevity, invisibility and possessiveness.
- Frodo - wore it for decades, longevity, invisibility and possessiveness.
- Sam - wore it for a few days, invisibility, possessiveness, delusions of grandeur.
While it's true that - aside from Sauron - we've never seen anybody of real power wearing it, we do have them talking a good deal about what they would do if they had it.
But - the key point is this: this could be part of the Ring's temptation. There's no evidence whatsoever that if they did get the Ring, they would be able to do what they think.
In other words: the Ring is useless to anyone but Sauron, but it tempts you into thinking it's a source of power, and in that way it gets you to wear it, and so gains control over you.
So while Gandalf may say "With that power I should have power too great and terrible" or Galadriel may say "In place of the Dark Lord you will set up a Queen" what's important is that it's Gandalf and Galadriel saying this, not Tolkien. As Tolkien said in a different context and of a different character:
Treebeard is a character in my story, not me...
And there's no reason why the same warning shouldn't apply to what other characters say too.
This is a lengthy preamble to help explain the next part, which is that it appears as though this is exactly what Tolkien intended the way things are to be.
Letter 246 is a good source of quotes to support this, and I'll pull a few from it. First of all, describing what "controlling the Ring" really means:
He needed time, much time, before he could control the Ring or (which in such a case is the same) before it could control him
Describing a confrontation between Sauron and a Ring-bearer:
Sauron would not have feared the Ring! It was his own and under his will.
Summarizing what I said above:
It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power.
The next may be read as a hypothetical contest between a Ring-bearing Gandalf and Sauron, but it's actually not; I've emphasised the relevant parts to highlight this:
Confrontation of Sauron alone, unaided, self to self was not contemplated. One can imagine the scene in which Gandalf, say, was placed in such a position.
And this may also be read in a similar manner, but again it's not; it's not Gandalf with the Ring but rather Sauron without the Ring:
On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession.
The final item is the reference to "Gandalf as Ring-Lord" but does that really mean a Gandalf that had mastered the Ring? What does being "Ring-Lord" actually mean anyway? To answer that we'll refer back to the first quote from this letter that I posted, and repeat it because it's worth doing so:
He needed time, much time, before he could control the Ring or (which in such a case is the same) before it could control him
So it's clear now that a so-called "Ring-Lord" is actually someone who had in reality fallen under control of the Ring, not the other way around.