I may not be a 20 year master mechanic (though I doubt that Yahoo poster was either, in truth), but I do know a bit about engines and fuel. The additives come in three types (four, if you count Ethanol as an additive).
Detergents (and corrosion inhibitors): These only provide "performance" if you have considerable build-up of contaminants in your fuel system and if used over time. That said, they tend to lower the energy density of the fuel, so you lose power today to potentially save power tomorrow. Let's call it a wash.
Octane boosters: The effect that these have on the fuel is literally labeled, and using a higher octane fuel than is necessary to prevent pre-detonation (pinging) doesn't provide additional benefit.
Lubricants: Like detergents, only provides a benefit when needed. Adding more lubricant than the minimum wont make your pistons extra slippery, it can actually gum them up (like adding more grease to a bearing than is necessary).
And as for Ethanol, the only benefit that provides is keeping farmers employed. It is less energy dense than regular gas and has corrosive properties that are harmful to rubber/plastic components.
Bottom line, I would feel much better about buying gas from a 3 year old WaWa station than a 20 year old Shell, as the biggest likely issue with the fuel would be contaminants that come from the old tanks/pumps. And I may be showing my age here, but does anyone else remember back when Shell wasn't a premium brand, it was actually the cheapo fuel station?
Ethanol sucks. That stuff screws up carburetors and fuel injectors.