Walla's last game as a forward was better than anything he produced in defence. Plus there's the whole defending thing in defence.
While that is true it was also a single game against poor opposition. I recall Jake Carlisle having a couple of dominant games up forward but in reality he was a more effective and consistent defender. Tippa would have to show a lot more consistency up forward to match the influence he is capable of having when breaking defensive lines. I hope he manages it but I certainly wouldn't be surprised if he is more effective down back.