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To elaborate on the why:
The difference between octanes is not necessarily more power, it's stability. Higher octanes are more resistant to preignition (knock, ping, detonation, etc), Which is where the mixture may ignite prematurely and momentarily force the crankshaft backward. This is bad, and could eventually lead to the engine beating itself to death. Higher octane ratings allow more aggressive tuning (spark timing, fuel ratio, higher compression, forced induction, etc) because the mixture will ignite when the spark is fired, not a moment before. The ECU has a window for some of these parameters, and it will reduce their values if it detects preignition. If it detects none, it will continue to improve it's parameters until it reaches it's programmed limit. More aggressive tuning is what makes more power, whereas a higher octane rating is simply what enables it.
As everyone has stated, for the 86, "premium" fuel (91+) is necessary for the engine to perform well. Lower octanes could preignite, and cause the ECU to retard timing and run a little richer.
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