FWIW, I refilled with 89 octane on the near-empty experience above. I thought I might need to haul a load of people through one of our mines and I usually use 89 higher octane for that since there's so much hill climbing. Ended up skipping the mine tour and just did my usual highway run and got 20.5mpg again over 380 miles. So no change I could tell in mpg, although I did put the hammer down a couple times to get around people and you really notice it there.
I've got a solid .7 mile on MAR1's personal best whoohoooo!!
I wonder what mpg I could get if I drove 65mpg and didn't have quite such aggressive nature about it...but that's not likely to happen. My other commute I do (50 miles of state routes, hilly, curves, short passing zones) I typically average 18.5mpg but if I drive like I have sense (or get stuck in a long line of traffic at 55-60mph....) my average jumps up to around 20mpg. I don't think I'd see that much of a jump on the highway though.