The Kockums air independent sterling system is interesting. I found myself wondering how it worked without any combustion. Turns out it doesn't do that but uses (probably catalytic?) combustion of liquefied oxygen (LOX) and kerosene to provide a 'hot side' and ocean water for the cold. Sterling engines are efficient; I built a tiny one as an undergrad that could run on the heat differential between a cup of coffee and room temp.
I'm a little surprised they didn't use a LOX/hydrogen fuel cell - but that's probably an indication of how little I know...
Oh, agreed that for Aus, range/speed is more important than absolute silence. You have to assume your opponent will be able to note when subs are in base, and where they are based. Slower speed means there are places and times that an opponent can determine that there's no possibility of a sub on station, simply because any potential boat would have to be in transit. Nukes don’t make that go away; higher speed reduces transit times, and longer tours mean at least one boat is starting from an unknown location.