Around here most gas stations all get there gas from the same place, the stations are just a franchise and pay for the "brand name" but sell the same gas as the independent down the street.
No.
There are Tier 1 refineries that produce Tier 1 wholesale gas ("raw" gas without the additives) and Tier 1 retail gas with the additives that meet the auto manufacturers specs.
In other words, the wholesale gas might meet Tier 1 wholesale gas specs, but without the additives at the retail level it doesn't meet the specs for retail Tier 1 gas.
When Tier 1 rating first came out, Shell premium "V-Power" was the cleanest burning and reduced carbon deposits more than any other gasoline on the market. That was a few years ago and it appears a few other brands of premium gas are the equal of Shell premium.
As the gas grades increase, the more/better additives are added to the gasoline.
I get approximately 1 mile to the gallon better with mid-grade vs regular in three 5.7 Vortec's and two 5.3 Gen III Vortec's. Time and again I have kept track of fuel mileage in one truck or the other and consistently they get better fuel mileage with mid-grade. Here in Colorado the octane rating is 2 points lower than in rest of the country because higher altitude/lower air pressure means the fuel molecules mix better with the air molecules and allegedly raises the octane.
Octane is a nebulous figure because fuel temp, humidity, and air pressure all affect the octane of gas. So the octane ratings are arrived at under controlled conditions and can vary a fair amount in the real world.
Edit: My bad. Originally the gas rated by the manufacturers was called "Tier 1", but it's now called Top Tier.