I have just written on the subject in the Brumbies thread. When I saw Siliva in both of his years in the Oz U/20s and for the Force As, when they were back east before the Super season, he struck me as a skills type of hooker that needed to add some hard-nosed play to his game. Nothing untoward in that: the same applied to Fitzpatrick at the same age and applies to a lot of young players and especially tight five forwards.
I'm sure it will come. It is better that a player is born with it like a Pocock, TPN or even the other U/20 hooker, the younger Roach, or Quirk, but many, many players acquire the attribute. He will have to.
Not to the point but this was one priceless benefit of the ARC: seeing which players were developing as hard-nosed players and comparing them to others of the same age. But it is an old song I sing.