Nice post Duke. Though I run my E320 on midgrade, I have never tried going to regular unleaded; however, you indicated that you can't tell the difference. I may try a tank load just to answer the same question for myself. I would caution anyone from doing this if they have a high compression engine that does not have knock sensors in the engine management system.
I would generally agree with your philosophy of using price as a guide with a few caveats. Though the additives put into gasoline are "roughly" equivalent, there are some marketers who do not additize their gasoline. Keep in mind that additization is not mandated by law or standard.......it's strickly dictated by the need to provide some sort of perceived marketing "edge" in the performance of gasoline.
You'll potentially run into this scenario buying gas from, for example, a supermarket chain that has decided to sell gasoline as well. They do not own (nor want to own) any production facilities. They purchase their product across our truck racks on a "cash and carry" basis. Many of these companies will choose not to add additives. They MAY NOT even know about additization, since gasoline marketing is not their core business! As absurd as that sounds, you would be surprised to find how many independent gasoline marketers do not additize their products.
Then there is octane. Not controlled by law or standard. The small business owner can take the risk of cutting cornors on octane. Who's to know if he puts a load of unleaded regular in his premium tank? Afterall, who is going to sue the little guy? On the other hand, everyone wants to sue the majors!
Then there are a number of independents that are not in the gasoline production business, yet care about the marketing of their image. Since they care about image, they are more apt to additize and accurately list the octane of their products. And, they can market their product at a lower cost than the big guys. I would not have a problem buying product from one of these mid-tier companies.
Well, at least my posts on this topic are getting shorter!