I have been building engines for a long while, two weekends ago I was assembling the valve train for a 6.0L GM Hi-Po LS engine. I have also in the 60's and 70's had cars with this thing called a distributor and multiple carbs and Sunoco 103 octane leaded gas. Hell, they would roll over and die trying to run on 93 octane. In other words I have some experience with engines. Have I modified a modern turbo car.....no. Have I owned them? Yes, a Buick V6, a Ford 4 and a Mazda 4, all turbo dinosaurs compared to the ST. My wrenching days are over other than bench work. But with 52 years of driving, rallying and AutoX along with maintaining my own cars I feel I may be more sensitive to how my engines feel and perform than most. And it is also possible since I have access to higher grades of fuel and am used to the way MY car runs on it I notice it more. The fact is on passing in 3rd gear there is a very noticeable loss of throttle response and acceleration on 87 octane in MY car. I put the emphasis on MY because all ST engines are not the same. Parts tolerance and break-in have a significant effect on power. So how my engine behaves will be slightly different than others. It may have lower power, higher power or an average power. The first time I passed on a short two lane road I immediately looked at the instrument panel to see if I had a "wrench" light on it pulled so badly on 87 octane compare to what I have become accustomed to seeing. I can't speak for any other ST but mine. Same with mileage, comparing ST's and different drivers experience is a lost cause in gas mileage. I can only speak to the data I have on MY car on gas mileage. I recently completed a 4500 mile road trip and closely monitored mileage and even brands of gas. I found several interesting things on gas, for example MY car runs slightly more efficiently on Shell premium than Chevron but best on a local oil companies 93 non-ethanol, I purposely tried to compare them as I was curious about the so called "top tier" fuels. I averaged 35 on the 93 octane Shell and Chevron with ethanol and 36.5 on Midnite Oil non ethanol 93 octane. That mileage on my ST by the way very closely tracks my 5 year experience with my Nissan Sentra that also required 93 octane on ethanol/non-ethanol 93. On the 87 ethanol my ST mileage dropped to a hair over 33 mpg at avg. 74 mph. That is a 2 mpg penalty so $2 per tank at $3.00 but the $2 does not offset the price difference in 87 vs 93 much less non-ethanol 93. So the money savings is there for those that want it. When I drove a tank of 87 where the speed limit was 80 it dropped to 31.5 mpg at avg. 83 mph with the 87 octane. Had another driver done the same trip with the same average speed the mileage would still be different due to driving styles alone. I tend to do a 1-2-4-6th shift pattern when not in a hurry but attack curves with lower gears when I can. If you try to drive the same way on 87 ethanol as you do on 93 non-ethanol pure logic will tell you after your engine management system dials back timing and whatever else it dials back it will alter performance. (per your owners manual) I am sure that if driven in normal commuter traffic one may not notice the difference. The difference may also be amplified with the MP-215 installed on my ST. I am retired and almost all of my ST miles are long trips so I do not do stop and go commuter traffic.
I see you have a supercharged car so I am sure you understand there is less potential energy in 87 ethanol than 93 non-ethanol, nothing is free, in our ST as tuned to extract a similar level of performance (hp) would require more fuel at 87 octane especially with a penalty of 10-15% of it as lower power ethanol, if the same level can even be achieved. I will pay the additional $7.70 a week at your 20K per year although here the difference is more like 40 cents a gallon here. I did not buy my ST to lose the power I paid for. If the $$ is the priority and you are happy with the way it works for you to run 87 octane you should by all means do so. But to claim the car runs the same seems ludicrous, how can it? It may run the "same" in commuter traffic but overall no. To make a statement that there is no difference in performance or gas mileage in your car says little about other peoples cars unless you they use theirs as a daily commuter in the same traffic pattern and are not able to or need to utilize the performance aspect. You may not notice it in your driving but I do in mine. I bought my ST for performance, I added the MP-215 for additional power. The fact that I can get 30 mpg in town and 35 or better on the highway negates any negative considerations I have on fuel prices. Thanks but no thanks on my ST running 87 ethanol when I have a choice. The two tanks of 87 octane I did run when I had no choice on were not a good experience for me or my car. I can swear I heard my Fifi thank me when I found 93 octane!
Different cars, drivers and goals mean there is no right and wrong on what gas you run in a car. What is right for one is wrong for another. That is why the ST is such a special car to all of us, it is a very flexible platform. From the time you order it or purchase it you make performance choices like the tires. Then we have mods, many will never do anything nor even look at a forum and others will will do their best to explore the limits of 96 CI. Other will walk a mild upgrade path. Some will require different tires due to conditions and some will only run max performance tires. Some will have very limited choices on fuel, others will have a wide choice.
As to the subject of 93 is bad which we know is not true and 87 is OK I feel any knock is not good, so running a fuel that knocks until the system re-tunes for the fuel cannot be good for a car period in my opinion. It could be Ford designed the pistons rings and lands to handle pre-ignition but to an old timer I would rather not have my car doing it. I have every intention to do well over 200K on this car and would like to never have to open the engine so I am anal on what oil and fuel goes into my engines. But that is my opinion.....and only an opinion as I also am not a knock expert but in my long experience I know of nobody that ever thought it was a good thing to hear in an engine. So again I will run 93 unless I have zero options. Wasn't there a gas that advertised "no nox" with Ethyl (lead) in the 50's and 60's? I looked it up...it was Gulf Oil No Nox! Wrench on!