1 - The additive packs can make a hell of a difference, especially over time.
2 - Many tuners have said time and again that Chevron has the best consistent octane ratings. Though I don't know if any of them have empirically verified this.
3 - Back in the 90s with my Jeep I delivered pizza for a summer. I compared Arco mpg to non-Arco. Arco was CLEARLY down on mpg, about 10% as I recall. I'm not sure why, maybe more highly oxygenated.
I also don't know if this still holds.
Anyway, sounds like Costco gas would be just peachy for the DDs at least and I will be on my way to setting a new rewards record.