Well from that, back of envelope I get if they mined around 6M...

Well from that, back of envelope I get if they mined around 6M tonnes over the life of the mine they would make an extra $120M before CAPEX by grinding the ore at 53 microns rather than 75 microns based on :

2.1% improvement in recovery by grinding smaller * 7 gram average head grade = 0.15 gram increase in recovery per tonne
extra $A25.00 a tonne in sales if recovery goes up 0.15 grams with POG at A$5200 an oz ($168 gram 0.15grams)

costs big assumptions BWI extra $3.00 in power cost per tonne by grinding smaller (10 BWI * $0.3Kwh + extra $1.50 in liner cost)
$25.00 in sales less $4.50 = cc $extra $20.00 a tonne advantage in finder grinding

What is interesting to me is the extra recovered ounces is not just a product of extra recovery % but also the deposit grade ie if this was a deposit at say 3 grams a tonne reserve but with similar recoveries to NN and Pepper , finer grinding would result in less than 0.06 grams recovered rather than 0.15 grams recovered , but the extra opex in finer grinding is the same for both the high and low grade deposits
Therefore it probably is worth it , providing CAPEX is not more than say $50M to get the improvement , but a 53 micron grind must be pushing the limits of a ball mill so would an ISA mill be needed and whats the cost of that ?

Welcome for the above to be corrected as am not a metallurgist