Ann: Premium Grade Water Soluble Sulphate of Potash Produced, page-5

It isn't about producing a more favourable end product, it is about producing more SOP from the brine by using the surplus sulphate. All the WA brines have a surplus of SO4. It is interesting to note that the K:SO4 ratio at Lake Way is significantly lower than at Lake Disappointment (6:25 v 6:30) yet RWD have never discussed the need for MOP conversion. Also note that KLL haven't gone down the MOP conversion route.

Yes, the MOP must be trucked in so transport costs will increase but those trucks will be carting SOP from the mine. The operating costs will go up but so will revenue because more SOP is produced.

This isn't new. It is used by Compass minerals in Utah. But remember evaporation is much lower in Utah so boosting output with MOP conversion probably makes sense.
APC are proposing the same conversion step and will boost their output by 50%. APC will be importing MOP into Geraldton, no doubt SO4 will do the same.

Currently SOP & MOP prices are linked due to the Mannheim producers costs of production. As MOP prices increase so do SOP prices. As a result, those SOP companies converting MOP are protected from rising MOP prices. However they aren't protected from lower cost brine or hard rock SOP producers.

Arguably every WA SOP hopeful should consider MOP conversion. APC have a fairly low grade brine and are constrained by lake surface area. MOP conversion makes sense as they don't appear to have the area for more ponds and remember lower grade equals bigger ponds and bigger ponds are needed for higher output.
SO4 at Lake Way have high grade brine however the diagrams in the scoping study suggest the lake area isn't massive. The majority of their brine will come from 130Km of trench (19/23GL). The diagram shows the proposed ponds, you can decide if they could squeeze in more ponds with the 130km of trench, I'm not sure they could without going off lake.