Why the hang-up with a/c speed which really will not make much difference, especially once you take into account t/o, landing, having a burger etc. The ETA difference is basically the time taken to drink another cup of coffee at Dubbo.
To put it into perspective. Let's assume you have the fuel and bladder capacity to fly YBAF-YPIR in one hit for a hare and tortoise exercise.
Distance = 976 NM
C206 - At 142kt, flight time = 5.58hrs
C172 - At 124kt, flight time = 6.5hrs
At 110kt, flight time = 7.43hrs
I would think a little extra time to rest and refresh at each stop will improve your safety factor. In that case there is really little difference to your ETA regardless of a/c.