Some good points but I wouldn't regard parity with the US$ as...

Some good points but I wouldn't regard parity with the US$ as the norm and anytrhing less as "foreign exchange gain".
It's just the conversion rate .
Also while there is a possibility of the A$ strengthening against the US$ (perhaps it's happening to a degree now ) due to the US devaluing their currency by all their debt and money printing there are other factors at play .
The biggest factor is China and the A$ very strongly linked to price paid for iron ore and for thermal coal .
China apart from the recent spat with Oz has set no targets and looks set for much weaker demand for their goods internationaly and domestically and their is a good chance the commodity prices will react to this and produce a lower A$ .
I'm no expert and wouldn't want to predict too much but expect a weaker A$ at least as likely as a stronger one over the next 12 months .
I found you're approach just a little simplistic for a complex situation and personally I still feel Australian gold producers in general one of the (if not THE) safest investments around .
Another scenario folowing your arguements is the US$ loses fiat currency status in which case GP bound to surge .

All very much IMO .