For my weight of 92kg and 184cm I use a powerful 70cm with the Apollo 07. In my mind the 75 as soon I am planing is to big (to much area) and limits my speed. I can imagine in constant low wind conditions when you are always on the limit of planing a 80cm can help. But much more efficient is a powerful tip (last 20cm) to produce the lift and tourge you need. Most fins are shaped much to flat in this section.
best regards peter