You won't notice a change in mileage with low octane fuel.
The problem you have in high octane fuels burn slower, increasing the detonation threshold. 87 burns fast, so fast it can cause detonation and pinging.

Back when we had the fuel shortage I was forced to put 87 in my crossfire. It absolutely hated it, It pinged, knocked, etc. It was horrible

Its also a really good way to inadvertently grenade a motor