It varies here, some areas have 91 and some have 93. If 100 is the norm for premium fuel at service stations in your area, there's a good chance that your stock ECU might be tuned for it (in which case the owner's manual would highly recommend only running 100 octane gasoline).
It is my understanding that the ECU has to be tuned for it for octane boost to have an effect though. All octane does is make it harder for the fuel to detonate from heat+pressure (before the spark sets it off), which allows you to run a leaner air:fuel ratio without experiencing engine knock/detonation, and a leaner mix will make more power. However, I don't think the engine can sense this and adjust for it automatically, so you'd just have extra insurance against knock without a tune for it.
On the other hand, someone here mentioned/linked a 370Z dyno result where they saw horsepower gains in the US from running some sort of 'oxygenated' 104 octane fuel. I'm not sure what that means - maybe oxygenated means the fuel has extra oxygen in it, effectively leaning out the mix from within the fuel and thus allowing the engine to take advantage of the 104 without knowing it.
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