I like BDC can literally pinpoint the first tank of the switch from summer to winter blend and back again as I too see a way off the scale increase and decrease in my mileage. It was bad before the outlawing of MTBE, but it is worse now with the E10-E15 blends we get in So. Cal.
I agree that there is a difference in extreme temperture changes, but in So. Cal. much of the time the difference between summer and winter is maybe 30 degrees, fall and spring are usually negligable. There is definately something there but it is often statistically irrelevant compared to the fuel blend differences.
I too lived in the bay area for a while while my wife and family were in So. Cal. back in the early 90's and my monthly drives up and down I5 were the times when I got my best mileage, 400 miles at a nearly constant speed made for excellent mileage. The most notable exceptions were during times of Tule Fog and holiday weekends when traffic would interrupt my cruise speeds.
Sure there is a power difference. My mustang gets more mpg and seat of the pants power with shell v-power. Knock sensors will retard timing if poor fuel is used thereby costing you power. And my mustang performs amazing off chevron 94 with no ethanol as compared to shell regular with 10% ethanol. Ethanol is pollution in our fuel causing consumption to go up due to poorer mileage as compared to actual pure gas. If they built cars to run off straight ethanol and the engines were tuned for it that would be a different story but as of right now that stuff sucks