Would upgrading Perth and Pearce to Cat111 be the cheapest way of solving Qantas' alternate dellemma for Perth...
Nope. Pearce is only 16nm north of Perth and the forecasts for the two are invariably the same. If the forecast for Perth is rubbish then Pearce will also be rubbish and not suitable for planning as an alternate, even with Cat III. By the way, Perth is already being upgraded to Cat III.