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During cruise (unless you cruise at 140 mph) your car is under such low load that it can run optimal timing advance with 87 octane easily. The factory tune may or may not do this since factory cars generally err on the side of less advance to help the cat work better.
Custom tune with lean burn stoich under light load and removed cat will net you noticeably improved fuel economy. I did that on my last DI car with factory wideband and EGT probe. Theoretically DI cars can run extremely lean with something called stratified combustion but that requires more than a simple remap.
Where high octane comes into play is under heavier load. And if you're getting into that range during normal driving your mileage will suffer anyway.
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