To be fair you might have left some horsepower on the table by not using premium. It is very clear in the owners manual that using less than 91 Octane MAY reduce performance.
Some people value extra performance(However small it may be) over $600. People pay hundreds if not thousands for exhaust systems that might give minimal performance increases at best. Do you tell them they are wasting their money?
There are some cars out there that actually recommend 87 or higher and say that higher octane fuel may be necessary for summer driving in the heat. For those cars I wouldn’t bother putting premium in them unless the temperatures are unreasonable and you are driving it hard.
For the Stinger I have always used 93 Octane because 91 Octane is not available in our state, it is 95+ degrees here for a couple of months each year, and I want my car to have maximum power every time I step on the gas.
My wife has the Ford Expedition with the turbo V6. Her car doesn’t have the same disclaimer about non optimal performance with lower octane fuel and it only recommends “87 or higher”. Therefore, I just put 87 in her car because performance isn’t a priority for me there.
I look at it this way. If performance is your priority over gas price and your manual recommends premium or just says that performance may be reduced with lower octane then you should use premium. Otherwise use 87.
Never EVER EVER use 89 or mid grade. No car is tuned for 89. If the car sees benefits from 89 then it will benefit even more from 91 but not necessarily from 93. Either 87 or 91+ but never anything in between.