Invite you to have a look at Matt Philip's recent NRC games. While he has been playing Lock, he is almost identical to Scott Fardy in size, and plays a very hard game. And while it might be a jump from NRC to Wallabies, he also had a very strong Super Rugby season with the Force. He is a year or two older than Hanigan/Dempsey and has a bit more experience at Super Rugby level. He has the pace to play No 6, a good ability to break tackles and imposes himself in contact the way we want to see our No 6 play. I have no doubt he would offer significantly more than Hanigan does, and would be more impactful that Dempsey who I rate much higher than Ned in any case. EDIT : forgot to mention, he is most capable in the lineout also, often being the go to jumper for the Spirit/Force.