The rhetorical function of Col. 1.15-20

I doubt that the NT epistles are woodenly structured according to the types of outlines for speeches given in the rhetorical handbook. Yet given the prevalence of rhetoric in the ancient world and the aural/oral nature of the New Testament's, rhetoric probably figured somewhat in the structure and function of various units. What rhetorical function does the hymn/poem of Col. 1.15-20 have then? Andrew Lincoln argues that it is part of the exordium and Ben Witherington places it in a narratio. I would be more inclined to see it akin to a propositio since it contains a summary of many of themes of the letter (see also Markus Barth and Margaret MacDonald who hold a similar view). But if Col. 1.15-20 is a preview of the theological contents of the letter how does that relate to the hymn's origin? Would a pre-written hymn/poem just happen to fit so neatly with Paul's exhortation to the Colossians? Or was the hymn/poem selected precisely because it was suited to the context of Colossae and perhaps embellished only slightly to make it more direct?