This sounds lovely, until you look at the noise footprints and transpose them; transposed noise means transposed flight paths, this would be 4 runways, but not independant, and ATC nightmare, I would suggest it would be no more efficient than the existing two bits of tar; with the exceptinon of the very Eastern strip with Northern departures in the southerly direction... Left turn out of the way...
This is the way to go, but not like that... It's also very ugly... Such is life... The environemtal loby would never allow anything to be built in the water, the impact of the 'greens' last time was a nightmare and that was just a bit extending into Botany Bay; imagine 10-20 times that? Remember Botany
Bay is very industrial, not exactly of the coast of a National Park now is it...
Not to mention the million dollar views disturbed with aircraft flying through them; The noise footprint with a sea breeze; right in the middle of major money suburbs I'd guess.
Bring any second major airport on; don't wait until you have reached capacity at SY; then it takes 5-7 years to do something about it, that's after constructuion starts.
I also agree with earlier posters that the second airport will not, in all likelihood, reduce the usage of the first; not unless domestic operators are given financial assistance (reason) to go there; combined with a high speed train network so that it's 30 mins to downtown; like Hong Kong (which isn't so high speed, but express).
Bottle of Rum