No need to waste money on high octane fuels. These are not high compression engines, so they do not require the higher octane.
Lots of factors come into play for the MPG, so saying you got worse by switching to a lower octane is not really accurate.
Sart/stops. Acceleration differences. Different roads. Different temps, different amount of idle time, etc will make a difference.
Now if you did this in a controlled environment with controlled fuels, then the results could be acepted.
I've used the same fuel on the same roads and vary from 11 to 14 MPG. It all depends on throttle, speed and idle time.
Engine masters on YouTube has a video where they tested different octane fuels on the same OEM style motor. No difference in HP/TQ to mention.