Halliday & Matthiessen (2014: 731n):
While some scholars have explored the possibility of grammatical metaphor within the textual metafunction, we do not see any evidence that the textual metafunction engenders metaphor. It is certainly a factor in the metaphorical mode of realisation – particularly, in ideational metaphor, as we have illustrated above; but the origins of metaphor lie in the need to re-construe experience (ideational) and to re-enact roles and relations (interpersonal). The role of the textual metafunction is of a different nature (cf. Matthiessen, 1992).