High compression engines require high octane to avoid knock. It's not about performance. An engine with a turbo or super charger will always need higher octane fuel.
Not all forced induction engines always need higher octane fuel. A lot of the newer turbo engines in cheaper vehicles such as Subarus are specifically rated for 87 octane gasoline. They don't knock.
If your engine does not require the higher octane, then no efficiency will be noticed and you're just burning money. If your engine is specified to take the higher octane, then you can notice an efficiency bump over running a lower octane fuel in most modern engines. The engine computer will adjust the valve timing to prevent predetonation with the lower octane fuel at the cost of efficiency.
Adding octane to fuel isn't adding a booster. It's adding stability to the fuel so it can be run in a higher compression engine. If your engine doesn't reach that pressure then you'll notice no effect except your wallet getting lighter.
That being said, unless you are constantly flooring the accelerator and / or doing a lot of track driving, it seems challenging to make a modern (and properly functioning) engine knock on a persistent basis (irrespective of octane).
This is a weird hill to die on man. Modern ECUs are smart enough to tune the timings in to prevent knock when the wrong fuel is used, at the cost of both efficiency and fuel economy. "Runs" is not the same thing as "runs well"
Are you familiar with Boyle's Law? You compress a gas and it heats? Higher compression of air/fuel lowers the detonation point. If your air/fuel mixture detonates at the wrong time in an engine, you will get damage. Higher octane fuels take higher temperatures to detonate.
That's why higher compression engines REQUIRE higher octane fuel, as the manufacturer will specify. Run without it, damage your engine and try to make a warranty claim. Good luck with that!
