IRR is simply the discount rate at which NPV = 0. They're both...

IRR is simply the discount rate at which NPV = 0. They're both functions of cash flow, so I don't know what you mean about NPV being more suitable.

No argument about cash flow being price sensitive. What's that phrase economists like to use? Ceteris paribus, and with what we know about grade and recovery improvements, I hope to see IRR improvements in that range. USD/AUD is heading in our favour too, as you've pointed out.