FWIW...
My youngest uncle - he's only 12 years older than me - spent his entire career as a petrochemical engineer. Worked 40-some years in refineries in Texas for Amoco, Texaco, Shell, Arco (AKA BP) and Valero. For the last 20 years or so he's been the 'head dude' in charge of gas quality & blending for Shell, though he's retiring imminently.
What he said, consistently, is that it's all about the same for 87 and 89 octane stuff. The only real difference is in the 'premium' 91+ levels where the additives really are different and the care taken is also different. THERE you will see a difference between name-brand and generic gas. FWIW, he'd also tell you to buy non-ethanol if at all possible, and that IF you want to buy premium gas, Texaco is the best blend and Shell a close 2nd. He once explained to me what "techron" is, but lost me about 30 seconds in... something about 'covalent polymerized' something or other... He never worked for Chevron but seemed to feel they were top quality. He did not have much respect for ARCO/BP quality: best I ever got out of him there was between the Brits being cheap and the Californians obsessing over meeting emissions, they never did do it just right.
Places like Costco, Giant, Turkey Hill, Sheetz, etc. that don't own their own refineries are buying from the big boys anyway... You may be getting amoco one week, shell the next, valero the following, getty the next, or a blend of them through a middle-man distributor. Certainly you're getting a 'blend' of them in the station's tanks quite often. But note that the big boys do NOT sell their own premium blend to the distributors/generics, they sell a different 91-93 to them that meets octane, but without as many additives, detergents and such.
He didn't regularly buy premium gas for his everyday cars. He did buy it for the 'toy' cars he kept with high-compression engines that need it. He did recommend running a tank of premium through a regular car 3-4 times a year just for the detergents. I asked if that didn't foul up the 'computer' and he said 'not enough to matter.' Never thought to ask him about Sea-Foam. Maybe I'll ring him up and see what he's got to say.
He did also regularly say buy gas at a high-volume station. To him the key difference was how long the stuff sat in the tanks ESPECIALLY with hydrophilic ethanol involved.
Just saying.