All you had to do was set out a 5 gallon red fuel container in the sun, right about when its 90 degrees outside, let it sit for a couple of hours.
Then remove the cap and take a look inside, you should see bubbles.
That would be the fuel boiling, do you know what happens when fuel boils?
It loses octane.
That could be why they keep the stuff underground at fuel stations.
I'm surprised you didn't see a decrease in mpg.
I'd have to guess it didn't get hot enough...
Now cooling it, that might do something.
But by then you're as well off cooling the intake air.
A FE gauge should be standard equipment in every vehicle.