We got our very best MPG in some ways (like the top half of the tank) when it was filled with high octane. I actually suspect this was a dealership trick and they "topped off" the tank to the moon.

I don't like being tricked, but I do want to prove to myself one way or another whether or not this was a trick or better MPG from high octane.

I don't think the dealership was trying to convince me high octane worked better. I think they were topping off to make it look like their hybrid was better than it is, but either way - we don't know unless we run a tank of high octane to see where things come out... making sure the tank isn't topped off.