Can't answer some of your Australia-specific questions, but I'll say that I've been pretty happy as a CX Gold, for the following reasons:

-- The seat guarantee will be quite valuable if you live in HK and like traveling during peak periods. A search should turn up some threads discussing this benefit to give you a better idea.

-- The soft service is pretty good. Onboard recognition seems to be becoming a little spottier, but they still sometimes come along with an extra bottle of water, or ask my meal choice ahead of other pax on long-haul flights. Seat blocking I have less experience with, and they've got a mixed record with that the times I've tried. And the one time I've found myself in a major irrops situation (knock on wood), they've come through pretty well.

-- You shouldn't count on op-ups, but they do happen and as a GO I've got a respectable track record. You'll be ahead of other oneworld elites.

-- If you're mainly flying CX and living in HK, huge advantages of being in AM include local card earning (check out our running thread on this), the ability to waitlist for AM awards, and the ability to use AM to upgrade if you prefer. Plus as a GO, you'll have higher priority on the waitlist.

The main knock on AsiaMiles/MarcoPolo is the poor earn/burn ratio. But it sounds like your main alternative, QF, is becoming worse. In your situation I'd probably stick with MPO as long as you get your credit cards set up. The earning boost you get from supermarkets, dining and so on partially makes up for the lack of tier bonuses on flights, IME.