I don't know about you but I didn't notice much of a performance gain (if any at all) or mileage gain w/ 91 premium over 87 regular unleaded that was equivalent to roughly an extra gallon of gasoline that premium cost you in price.
That said, I've been told there is definitely a difference when you have to smog your vehicle though. Premium + fresh oil change was what worked for me.
Can you elaborate on your 91 "test"? I say "test" because without a doubt, this is not a dispute, and I'm having trouble understanding how/why this is treated as such in discussions on this forum. The actual mapping of the ECU lays out in front of anyone that looks at it, the timing will advance as far as it can until pre-ignition is found (among other variables).
Now, I hope you dont mind but let me lay out a few scenarios to better break down what can happen when someone considers written code in an ecu subject to a 'verification test'
Please read these with a sense of humor in mind, but what I'm about to tell you is true and has happened. - Some folks reset or dont reset the ecu and add higher octane to the vehicle, then proceed to drive as normal for a period of time, usually a full tank and record their data. Often then decide it's not enough of a change, scrap the idea and move on with life.
Other folks may reset the ecu, drive the car for a month and only notice a slight increase in MPG and maybe, like folks in the southwest where 91 is the limit most the time, dont see enough to offset the cost.
Others, even more thorough, might reset the ecu, make sure the air filter is new, fuel filter is fresh, spark plugs and oil are changed or at least clean, drive for a full year, record data, see a justifiable increase in MPG and power, decide to keep going or give up because it's a nominal difference.
Now here's the real over achievers, let me preface this with a 'yes folks, this has happened' in regards to the mitsu V6 engines and the ecu discussions over the years; You have to remember, this engine is used in a lot of platforms and in many balls-out high RPM set ups long before someone in smog capital of the US decided to conduct a 'test' with a giant SUV that's fighting to breathe through a snorkel intake and 4 cat's stuffing up it's exhaust...
people have done all of the previous check list of filters and fluids etc, then gone as far as completely rebuilding the engine to eliminate the years of sludge built up inside from running conventional oils and low octane fuels, then started fresh and recorded their data, and to no ones surprise, these cars typically saw more power and mpg than the run of the mill, daily driven counterparts.
Did you know that excessive lifter tick can cause 'phantom knock' and force the ECU to pull timing on an otherwise healthy running engine?
What I'm getting at here is; there are a lot of variables in the real world that can affect your perception of something, and if you dont want to run premium, dont, in fact, it's no one's business how you treat your engine... but it's not up for debate; higher octane yields more power and more power will provide better fuel economy from an efficient running engine.
If you'd like to conduct a real test, you need to get access to a dyno and a datalogger, until then, you guys are literally blowing smoke up your own backsides. I'm sorry, I know others on here come from an automotive performance background and they are probably struggling with how to explain this politely to everyone else too.