I noticed in the news that Tilt Energy are developing a PHES in...

I noticed in the news that Tilt Energy are developing a PHES in North Adelaide at the sight of the old Highbury Quarry.

After a bit of searching I found this bit of research.

Note the Capex Numbers  $250m vs $430m
The MW capacities are 220 vs 300. So comparing apples with apples the Cap ex comparison is 
$250m vs $315 Difference = $65m
This is muddy however because Highbury only has 4.5 hours storage compared to our 6 hrs
So on a total MWh the capex comparison is $250m vs $420 Difference = $170m

How valuable is the extra hour and a half ? $50m might be a bit ambitious ??

Notice the Snowy has enough storage to run for 175 hours. This probably explains the big Capex 
Could it be designed to add more turbines and kick the capacity from 2000MW to 60000 MW. I guess when money grows on trees and you don't have any earnings hurdles to jump Governments can afford to do that.