Australian market M103 had the most retarded timing of all of them. I have a feeling that they don't develop the same power as the US/Euro cars

Look at this chart:
http://www.w124performance.com/service/w124CD1/Program/Engine/103/15-0515.pdf

What is the situation with high octane gas down there? I've never been to Australia.

Anyway if you want more power and you have access to high octane gas then you can remove the reference resistor for the EZL ignition module and advance your timing. But be careful when doing that and always run high octane gas because these motors don't have knock sensors.

look at page 3 of this pdf. That's where it's located on your car.

http://www.w124performance.com/service/w124CD1/Program/Engine/103/15-0520.pdf

Also another tip: What are your alternator voltage regulator brushes like? The regulator with brushes is replaceable.