If Sirro moves to the back row, it will almost certainly be to the edge, not lock.
But addressing the OP, currently Sirro is a 5/8, just about all the best kids in juniors play five eight, halfback or fullback. Many of which move elsewhere. Frank Puletua played a heap of 5/8 in rep teams, to pick an obscure example, Luke Burt was an amazing 5/8 for Newcastle in HM.
Sirro's skill set IMO is well suited to an edge back rower, particularly if his path to the halves is blocked by others. His strength is running the ball and he plays very direct, he clearly has the frame, this could be well used in the back row. The NYC game afainst parramatta, he was very strong in teh backrow.