If you price it on throughput and not storage size, because Snowy 2 is very constrained on when it can charge and discharge then the realistic 8Twh per year you can get out of Snowy2 loses to batteries by a factor of 3 based on your costs.

But that's assuming the batteries can charge and discharge in full every day.

The truth is probably somewhere in the middle where Snowy can act as seasonal storage with a big capacity that isn't used every day.

I think it would then also lose on price if compared with a portfolio of batteries alongside solar and wind to directly supply electricity and continually recharge the batteries over seasonal scales but that is a more complex comparison.

In general I think even good hydro schemes (and Snowy 2 is not a good one) rely on non-electrical factors to be worthwhile now batteries have plummeted in price, e.g. managing seasonal water flows for agriculture, and the electricity can be seen as an additional benefit.