In case you missed it:
Size of the 4 is a deal breaker. The back seat is even smaller than my ISF, which is already pretty small. It's pretty small for me as well. But I only have kids that would be back there. Do you all have friends or something? :D Why the need for large back seats?
www.cadillacvnet.com
To add a little backstory to it: I never considered Costco gas a few years ago because at that time, they did not have top tier gas, which they now call Kirkland Signature gas. However they were slowly changing the stations over to it, and now I believe all of their gas stations only carry Kirkland signature. That's when I started my research and if their gas competes with the best of them.
Nowadays with direct injection engines, it's more important than ever to use Top Tier gas. You don't have the cleaning effect of port injected gas being able to reach and coat every part in the combustion chamber before the air/fuel mixture is ignited. Direct injection engines have an issue where your fuel injectors inject fuel, and it's ignited immediately. There's not much time, if any, for fuel to clean the injectors themselves, or the spark plugs.
And if you use gas that isn't top tier? You compound that problem and lose efficiency in the long term.
This is why some manufacturers like Toyota and Ford coming out with dual-injection systems to combat this problem to get the cleaning effect of port injection, and the efficiency of direct injection.
Many manufacturers also recommend top tier gas(which like someone else said, is just a marketing terms that means extra detergents added above what the epa requires).