Why is that a myth?
Food conversion is simply the amount of feed to the amount of lbs grown.
Higher quality feed drives a higher price to produce. 2 years ago, I spent the investment on "higher quality" feed that guaranteed a 1:1 ratio, one lbs of feed to one lbs of fish. It was a closed loop system with good water quality. I kept temps within a 3 degree margin year round. The fish were sparsely populated in relation to fish/gallon, as I didn't want to worry about crowding. The fish DID NOT attain the growth guaranteed.
Conclusion: Feed producers are in the business of selling feed, not growing fish!
It's gotten me better yeilds per fish to look up the protein %'s for best results of a given species at various stages of development and use "alternative" feeds and own recipies. Also, cheaper to produce feed.