I was reluctant to post on this subject well mainly for that reason. Its subjective. One example I can give that resulted in hundreds of data logs of my previous car a Lancer Evo MR that was tuned for track duty, but was also my DD car.
I did a study of my own due to the HP variances I kept seeing on the dyno. That in itself is subjective. I used an OBDII USB device and a program called EVO SCAN to data log my car. This showed, basically everything your engine reads. I was looking closely at knock.
On the dyno HP would be down slight whenever knock went up, and I noticed a trend on certain fuels on dyno days and put the 2 together. So for one month per fuel brand which was about 4-6 fill ups for DD duty I recorded knock readings from driving to work, WOT and WOT on the dyno. I did this for the following brands of gas.
Chevron
Union 76
Texaco
Shell
All fuel of course was premium unleaded 91 octane. Of the 4 Union 76 showed the least amount of knock which netted more HP and TQ on the dyno (consistently) in my particular application.
Now, does different brand of gas decrease or increase HP or MPG's is VERY SUBJECTIVE, BUT if you look at knock, gas quality comes into play. This in itself is subjective due to variables such as:
Age of gas station tanks
water contamination
service of pumps
car maintenance
car tuning
driving style or habits
any of the above reasons may cause knock. Knock retards the timing in your car, and in forced induction reduced boost levels. These factors would indeed reduce HP and TQ not to mention MPG's.
In closing, my report took over 4 months and hundreds of data logs. I do believe that some additives burn off better than others, and I do believe that fuel does vary in quality even within the same brand. I went to one Chevron station had a higher value of knock, vs another with less.
So in the end, I believe gas quality and gas contamination is key in vehicle performance regardless of brand.
For personal preference though, I do avoid ARCO and ANY unbranded gas like COSTCO. ARCO I had bad experiences with many times, and unbranded gas? Well I just like to know what I am buying. May sound silly, but if my car manual recommends High Detergent Fuel, and I buy unbranded gas, how do I know its High detergent? You dont. Is why I avoid them.
Everything else is Subjective.
Just my ten cents.
Jeff