Elevation effects detonation. Lower altitude makes it worse. I would go colder on the plugs. No matter what you read, if you don't try it, it doesn't matter.
...And after looking it up, where he was is actually higher in a large portion of places. Rancho Cucamonga elevation according to wiki, ranges from 1400-3000'...San Antonio 772' average, and Abilene is 1780'. I know also, DA can play effectively play with that number, but I'm much more inclined to conclude the folks in Cali buy different blended fuels due to the Air Resources Board out there.
...more specifically, California blends removed MTBE, which last time I recall any reading about it, reduces potential for preignition.
