At sea level, the GT-S won't achieve max ambient pressure at BDC unless the throttle is wide open and the RPM is high enough to switch to the second cam profile. The second cam profile allows the valves to open a little more, thereby letting the engine breath a little easier. The second profile kicks in at 6,000 RPM. So 87 and 89 octane are probably fine for normal driving at any altitude, but below 10,000ft, you need 91+ octane when the RPM gets to 6,000 at full throttle.
If you use 87 or 89 in your GT-S, do not rev the engine to 6,000+ RPM.
This is why manufacturers specify minimum octane requirements, it doesn't need it most of the time but there's so many nuances to the physics, it's easier to just say "91 octane or higher" and never risk knocking the engine than to explain all the different scenarios where a lower octane is acceptable.
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