I've got an M1 with an A10. Over the first 3000 miles, I've averaged 22.5 MPG on 87 octane. Probably half of that is highway driving at 75-80 MPH. Where I live 93 octane run $0.70 to $1.00 more per gallon than 87 octane. I fail to see the cost savings if I would switch.
I took mine to a Mustangs and Mustangs car show recently with the gloss black spoiler. Most everyone I talked to liked the gloss black,, as it matched the rear taillight panel. Most said they preferred it to the magnetic. When you consider the other things that are black, mirrors, moldings...
I had an 18 GT premium before purchasing my Mach 1. Put 40000 miles on it, all on 87 octane. No issues using name brand gas, like Chevron, Shell, or Sunoco. Running the Mach 1 on 87 octane also. Now, if I was tracking it, I probably would change the grade a bit.