I did the math over the course of two months.
Using 93 octane from Shell or BP in the GX improves overall mileage enough to actually save 1.3 cents per mile in fuel cost. But even if it cost a little more per mile, my butt dyno says the engine runs noticeably better with hi-test.
I agree with Paul. I have a premium vehicle. I'm going to use the proper fuel.
Once when I was driving my Z28 as a daily driver, I got cheap and put regular in it. I hadn't gone 2 miles before it started sounding like someone was shuffling a deck of cards under the hood. I never got cheap on gas again.