tru-boost: idk about 50hp on the gen 3 coyote, only seen an ecoboost dynoed with 87 vs 93 and it was just under 50 at the peak but for the most part was the same until 5k rpms where the 93 kept pulling notably harder. I think boost needs the octane more with the extra power trying to be had though, so i'll call boost a variable in the equation.
I admit I can feel a slight difference between 87 and 93 in the mustang, i have done A/B testing where i ran a couple tanks of 87, ran it dry, then 93, ran it dry, then 87, ran it dry, then 93, and on and on for a season of driving. 93 gave a little more mid-range response and pull, little better off the line, and even some more top end. Overall it felt stronger and it felt like it handled the summer heat better too because it didn't seem to pull timing as much as it got hotter and heat soaked. I also did 0-60 testing on the same road/similar weather with 2.5k clutch dumps and the 93 consistenly ran faster with the same tires and within 500 miles of driving between tests. I can personally vouch it made a performance difference, but not in rush hour traffic...same same there.
No doubt there is a performance advantage. heck i even got over 17mpg vs about 16 by going to 93 on it. but maybe i drove more hills or hit less traffic, idk.
But simply saying because its 12:1 that you can't is not true though. Or that you outright cannot run 87.
Is it a great idea? i'd say no, but that's subjective.
But my mustang has an 18 gallon tank and premium is nearly a dollar a gallon more where I am and I have a 30 mile commute one way to work. So 16-18 gallons a tank, times 3 or 4 fill ups a week (including weekend) isn't just $5. If I can afford it makes no difference though, it's already just a summer toy/summer daily for me so its not a real issue, but I think we can at least agree that you're leaving some
performance on the table - how much, well you can pay for the dyno time and I'll bring the car. If its worth it is still up to the individual to decide based on their use case. Though I really don't think people should be buying cars they have to ask about things they can skimp on doing. I also don't think the BRZ/86 will lose the same 50hp necessarily but maybe it would; not enough data and no one's done the testing.
EB dyno sheet comparing 93 to 87 on stock tune as tested by Mishimoto. Yes I cannot screencap for shit, i only saved this as a reference for myself.