The Costco by me has such high tank turnover, I'm honestly surprised that the Shell gas seemingly did better. When I first bought the car, I only used Shell. When I switched over to Costco, I consistently got 1-2 mpg better. Go figure.
I'm curious if Stratified has ever dyno'd the OTS Cobb maps? Any idea of how much extra HP the 91 oct maps give? Cobb has dynos listed for the 93 oct maps, but nothing for 91 oct.