Interestingly, when Sha'ul wrote his letters, I don't think he considered them Scripture, but rather simply midrash, commentary on the Scriptures--which he would have considered to be the Tanakh and (insofar as they were written and available) the Gospel accounts. I have looked in the scripture for clues on this subject. I feel that at least some of the authors of scripture knew that they were writing inspired words. John, writing the book of Revelation, should have suspected from the first verse.
I find it interesting that you quoted 2 Pt. 3:15-16, which I think suggests that Peter knew that Paul’s epistles were indeed scripture, He accused some, in his audience, of treating them like they treated other scripture. Why would this be a problem if they were not inspired?
I never said that Sha'ul's letters aren't Scripture or should be removed from the canon, and clearly Kefa considered them to be inspired. I just doubt that Sha'ul knew when he was penning them that they would turn out to be so. He clearly wrote them as commentary on the existing Scriptures, which means that they can't simply be read as stand-alone pieces if you want to understand his real points. Further, with the exception of a few, like Romans, in which he was simply expounding doctrine in general, all of his epistles were written to deal with some crisis or another that we aren't given the full details on, so we have to be careful not to misunderstand the one-half of a conversation that is recorded for us.
Put it this way: If you read only the Torah, and rarely if ever touched on the Prophets, Writings, or New Testament, wouldn't you likely develop some incorrect theologies because of the lack of balance? Well, what about people who develop 90% of their theologies and sermons from Sha'ul's letters, then?
I'm just arguing for context and balance, that's all.