[Boromir88]

Here is the Letter SpM is talking about, Letter 246:

In any case a confrontation of Frodo and Sauron would soon have taken place, if the Ring was intact. Its result was inevitable. Frodo would have been utterly overthrown: crushed to dust, or preserved in torment as a gibbering slave. Sauron would not have feared the Ring! It was his own and under his will. Even from afar he had an effect upon it, to make it work for its return to himsefl. In his actual presence none but very few of equal stature could have hoped to withhold it from him. Of "mortals" no one, not even Aragorn. In the contest with the Palantir Aragorn was the rightful owner. Also the contest took place at a distance, and in a tale which allows the incarnation of great spirits in a physical and destructivle form their power must be far greater when actually physically present. Sauron should be thought of as very terrible. The form that he took was that of a man of more than human stature, but not gigantic. In his earlier incarnation he was able to veil his power (as Gandalf did) and could appear as a commanding figure of great strength of body and supremely royal demeanour and countenance.

Of the others only Gandalf might be expected to master him - being an emissary of the Powers and a creature of the same order, an immortal spirit taking a visible physical form. In the "Mirror of Galadriel", it appears that Galadriel conceived of herself as capable of wielding the Ring and supplanting the Dark Lord. IF so, so also were the other guardians of the Three, especially Elrond. But this is another matter. It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power. But this the Great had well considered and had rejected, as is seen in Elrond's words at the Council. Galadriel's rejection of the temptation was founded upon previous thought and resolve. In any case Elrond or Galadriel would have proceeded in the policy now adopted by Sauron: they would have built up an empire with great and absolutely subservient generals and armies and engines of war, until they could challenge Sauron and destroy him by force. Confrontation of Sauron alone, unaided, self to self, was not contemplated

The first paragraph makes clear that no "mortal" would be a challenge for Sauron, not even Aragorn. And it explains how Aragorn was able to challegen Sauron in the palantir. The 2nd paragraph is what SpM talks about, how he might be the only one expected to beat Sauron, if he had the ring.

SpM I agree with you on many points you have made. First, the one I just said in the paragraphs before.

The Ring would do its utmost to create instability within Minas Tirith and Gondor so as to make it easier for its Master to reclaim it.

Very true, especially if the ring was in the hands of Boromir, I will have to say, there is a high possibility of a kin-strife. At first, I thought he would have given it to Denethor, but now reading your post, it makes a lot more sense. The Ring would strive to rip Gondor apart, and Boromir just doesn't have that same "will" as Faramir, he wouldn't hand it over.