Interesting points ...
It used to be a pretty straightforward topic ... if your car was running properly and it pinged, you needed higher octane.
Then came EGR and other mechanical issues and problems led to pinging with the correct gasoline and some folks then simply went to the next grade up. Should have fixed problem.
Issue now I think is how flexible the entire engine is to changes related to fuel octanes ... if your engine management system is working properly, many engines will accept a broad range of octanes, but performance (power) will suffer ... questions, though are: 1. does the mileage also suffer, and if so, how much relative to the marginal decrease in the cost of the regular versus "designed for" octane; 2. is the engine, with timing retarded and other engine functions modified to prevent preignition, being damaged in any way?
Data on this would be very useful ... while I have seen a few articles on this in magazines, don't know if anyone has done a thorough test with various types of engine management systems to see which ones can adjust to lower octane, which ones do it better than others, and whether or not this is an OK thing to do, given the other factors.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg