Concerning Psalm 89:28, Marvin Tate in Word Biblical Commentary vol.20
(Word, Dallas 1990) quotes T.N.D. Mettinger, "King and Messiah...",
(Gleerup, Lund 1976) p.268 in saying that this verse "refers to a
status 'bestowed upon the king by a divine act of will, and not of a
right held by descent': 'I will make him my firstborn' - David is
Yahweh's 'firstborn'!... The supreme expression of a king's position
is that of a BKWR ('firstborn')-status (Mettinger, 263)." I would
suggest that this is the sense in focus in Col. 1:15.