MPG around town was about 18mpg and 28-32mpg on the Hwy doing 70mph. If I ran 80-85 I would get 25-28mpg. So not that much difference. My goal was zero knock and that was achieved.
I was going to buy 80s for mine. It's the last major piece of the puzzle before I'm driving it! What are your duty cycles like? Do you still have some room to grow?