You don't say how long you are planning on this trip taking.
Your biggest problem would be the weather
, particularly along the NSW coast where it may be necessary to wait several weeks for a good weather window.
With two 255hp petrol engines I would have thought that your most efficient and reliable speed would be well in excess of 8 knots.
You'll need to do some time trials to establish your optimum power settings. With the amount of fuel that you would burn some fancy fuel measuring hardware
could pay for itself many times over.
A quick search suggests that your top speed may be around the 20 knot
mark but your best cruising speed could be a fair bit less than this. You'll need to do your own navigation
, but as an example, if the distance between ports
is 100nm your trip time may be measured in hours, rather than days. It could be possible to do some legs using the sometimes calmer morning weather, starting early and having lunch at your destination
It may be possible to reduce the fuel usage by removing as much of the top hamper as possible.
When I was checking your top speed I found the Prop Scan website. They claimed increased efficiency and speed if they reset and balanced the propellers.
And finally I strongly urge that you wear lifejackets at all times offshore
, preferably those with inbuilt harnesses so that you can be tethered.