As noted, the octane rating of a fuel only relates to it's resistance to detonate and doesn't indicate one has more "energy" than another. For an engine that is knock limited in regards to MBT (which stands for "maximum brake torque timing" or "minimum timing for best torque", and is the ignition timing where the engine makes maximum torque and any timing advance lower or higher will produce less engine torque), then running a higher octane fuel will allow you to take advantage of increased timing to get closer to the MBT timing and is where the extra power comes from, if the computer is calibrated to take advantage of that high octane fuel.

Most modern engine controllers operate engines right on the verge of knock for best performance (again, if the engine is knock limited, or in other words the best timing or MBT is at a point higher than the fuel octane will support then getting as much timing as possible prior to knock will provide best power). For a stock engine this is perfectly normal and should not impact long-term reliability. For a stock car, especially one with a reasonable compression ratio and specific output (hp/L), if the manufacturer says you can run regular grade gasoline then you'll be fine. They do a ton of testing while doing calibration development as well as engine dyno and real-world testing and the powertrain has to meet the same durability requirements. You'll give up some power but you're not going to kill anything prematurely.

Now the cost of fuel has increased significantly, but the price difference per gallon from the lowest grade to pump premium is still about the same, figure about $.20-.30/gallon at most stations across the US. That's a couple bucks per fill up and for someone who tops up the tank a couple time a week you can figure it ends up being "only" a couple hundred dollars difference per year. If you buy a performance vehicle is that too much extra to pay to extract maximum power? Is that "only a couple hundred bucks" that important to save? That's up to each person to decide.

Also one other thing to note, when the manufacturers are doing calibration testing and recommend a fuel grade, it's usually what's best under the most severe operating conditions a vehicle will see. So for someone living in cooler climates, not hauling around a lot of weight, etc., it's quite possible some cars that state you should run premium but mid-grade or lower is acceptable will run the exact same with no performance difference. Start driving up mountain passes and other hard driving conditions and you might see the difference. And some cars will be noticeable no matter what driving conditions and are very octane dependant with respects to their output. Every car is going to be different so the best way to tell for certain would be to monitor timing and knock on a scan tool/scan gauge. Then you will know how the fuel is affecting how the car is running, not just guessing and relying on the often inaccurate seat of the pants measurements.

 

GM recommends at least 91 octane in the 2.4L VVT.......which includes your GF's all stock automatic.