You never receive 100% of the spot NP for concentrate sales. Using the same inputs, the margin would be more like ~AU$3.00/lb -- better than presently, but not AU$6.00/lb.
(Calcs: US$7.50 (AU$11.19) * ~70-75% payable = say AU$8.20 (range of AU$7.85/lb (70%) to AU$8.40/lb (75%)), less US$3.50 (AU$5.22) = AU$3.00/lb).