[dhorvath]

I have using regular since the car was purchased. I may try a tank of premium just to see if there is any perceptible difference in performance....

You may need several tanks.
1) tank isn't really empty so premium is diluted.
2) computer takes time to relearn

 

[dhorvath]

The engine is most efficient right >< at the ragged edge of pinging events. The only way for the computer to determine this is to vary mixture/ignition timing real time as conditions change until ping is detected and then back off (adjust the mixture/timing). That is happening constantly no matter what fuel/octane you're running.