Right, it's not about fuel economy nor fuel efficiency. It's about how the engine was designed and the ECU tuned.
OEMs know some people will put anything that vaguely smells like gas in the tank so they try to protect the engines. Some do it better than others. Too low of octane for a particular engine (note that "too low" is a dynamic thing - depends on a huge variety of factors including the combustion chamber pressure (which is much higher with FI engines) and temp at each moment, spark plug temp, exhaust valve temp, etc etc) will result in the ECU retarding timing. Some handle this more gracefully than others. E.g., my '02 Lincoln simply cannot pull enough timing to run reliably on <91 octane, so it'll have detonation which will destroy rod bearings.
Best case, the ECU pulls enough timing to prevent detonation. It'll constantly be in "Oh Shit" mode and you'll be leaving power on the table. Worst case, the ECU can't keep up with the changing dynamics and you end up with rough running, detonation, and unhappy endings.
The good news is the Stinger ECU is super agressive about adjusting timing. They actually use timing adjustment to modify power output - timing advance drops to about 0degrees during shifts to be nice to the trans. So it's likely to be just fine running on the cheapest of gas, but the ECU will likely mess up from time to time and try to get back to its baseline tune, realize the engine is rattling apart, and back off again. Especially when flooring it.
Note that detonation/ping is different than it was in the '70s. We relied on the "metal can of rattling marbles" sound to tune ignition curves, but by the time you hear the marbles the problem is already extremely bad. Modern systems use a knock sensor and apply rather advanced signal processing to it (it's literally a wound wire that vibrates) to determine when detonation occurs. Note that different ECUs have different signal processors and algorithms. Ford ECUs from the late 90's were awful at this, so most tuners turn off the factory knock sensors.
