Matai a finesse player? Hahaha.
As for Creagh/Wiki/Sonny Bill. Creagh didn't play for very long as a winger. He was an 18 year old who was a f*cking athlete and the Dragons wanted to get him onto the field in anyway possible. At his age (and still growing) it wasn't a problem to put him on the wing, and even this year he shifted back to the centres for a few games. But he was always a forward playing in the backs, whereas Matai (to me) is a back playing in the backs.
Wiki spent quite a few years at centre, and has gradually moved from the centres, to the backrow, to the frontrow, as his body has changed. It was almost as if he changed position as a result of his body changing, whereas moving Matai and asking him to put on 10 kgs would be changing his body so that he could change position.
Sonny Bill is exactly the same sort of case as Creagh. A young forward playing in the backs so that you could get him onto the park - but always destined for a lifetime as a forward.
After all that, I think Matai could work in the backrow. If anything I'd want Willow to start and get more minutes, and have Matai come on as a relieving bench player who can replace Willow, and rip in off the bench. At least for his first season in the forwards. Something like that. I don't know. None of these options are ideal, so it'll make for interesting debate in the off-season and to see what Des finally settles on.