In general my experience with mileage is the opposite. For 9 years I lived in Vegas which is relatively higher altitude (2-3k feet) and my LC always ran best with 89 octane. Now I'm at sea level, 89 octane still seems the best, but what was surprising is trying a full 91 octane tank of Shell gas, seeing my mileage drop from ~14mpg consistently to what is probably running closer to 11mpg. That's over a 20% drop!
For any opinions that O2 sensors just go bad over time, even without throwing a code, I find other opinions that say just wait until they actually go bad for real. But at this rate I could save the cost of O2 sensors in gas in just a couple months if I consistently got back a little mpg. Heck, the wasted gas in this tank alone at this rate will be $20.
The 87/92 thing has been kicked around here almost as much as tires. This is the first I've heard of 92 causing a hit. Not quite sure how that happens as the fuel energy is the same. The problem with the 87 is that the engine management can retard the timing in an effort to inhibit knock which comes at a price to economy.
There have been other posts here about mileage drop on the LC's being a precursor to O2 failure. The theory is that the sensor isn't out of spec yet still messes fuel trim long before it finally throws a code.
Lastly, to those who've used the Amazon Denso sensors... any feedback? Same performance as OEM? I know we discussed the non-OEM Denso units as not necessarily matching Toyota spec even with the same part number.