I also replaced top two battens to super soft to maximize sail draft. In combination with relatively stiff mast my FO1 looked really powerful.
I had two planks: 46mm and 42mm bend. Here the choice was easy as well: go for stiffer one. In light breeze stiffer plank will force mast to bend more and this is what I wanted especially for all difficult downwind legs but the trade off was hikes if breeze picks up. Having less responsive plank I could immediately feel more pressure being loaded on the mast. Perfect.