3 hours would be a minimum - I would never book anything closer than that, even on the same PNR.
Just a point on the new TBIT - I arrived there a month ago. It was the QF11, no other flights had arrived, one of the first off. So it was quiet - got through the passport area within 10 minutes but then spent 45 minutes getting out after the bags with customs. Of course flights from Japan arrived about an hour earlier so they were 'processing' those people. Then a Korean A380 arrive and the queue continued to grow
So from landing to the kerb took 1 hour 10 minutes - the same as it normally does, but usually the 45 minutes is spent at passport control.
I will reserve my judgement with more trips but at the moment, even with the improved terminal, it isn't any faster.