Sounds like you have been the victim of the lets try this, oh, that didn't work, let's try this, dealership mentality.
Has anyone done any exhaust gas analysis on the car?
Has anyone tested the fuel pressure and flow rate?
These things might seem basic, but an accurate 4-gas analysis of the exhaust, combined with individual cylinder fuel mixture testing, and high tension spark analysis, will help to pinpoint the area of the problem. I would be prepared to back my judgement (and my equipment)to the extent that if I couldn't accurately diagnose your problem, then I would not charge you a thing.
My workshop is at Penrith, if you get desperate and want to drive that far, (I'm assuming that you live somewhere close to Ryde),I would be happy to check it out for you.