The general theory of its all brewed at BP Kwinana is correct.
It's then piped to the other branding transfer terminals where its dosed with the "special" additives to make it shell or caltex or what ever.

Todays useless fact: Shell and caltex share a terminal in freo.
Same tanks, same pipes and the only difference is additives that are added at the loading arm just prior to the truck.

I would be very surprised if there is any major difference with any brand of fuel.
I've never noticed any difference in economy that could be any thing other than my right foot.
Just pick the biggest busiest servo you can as the fuel will fresher.
Bigger servo= more customers= loaded with fresh fuel more often.