isn't the "performance" the manual is referring to acceleration (hp)? you'll note that their advertised 0-60 time was using 93 oct.

i doubt there will be much, if any, difference in mileage, all things equal- but i'm not enough of a gearhead to know for sure. the higher octane gas will allow advanced timing so i'm guessing the ecu adapts, but this doesn't happen immediately. i'm not sure what else the ecu does for better performance. it would be great if someone could expand on this.