The higher the octane, the more controlled the burn (slower, less explosive) so added timing/compression/boost can be run. On a NA engine or one the tune is not set to utilize the added octane anything above optimum for the engine/tune will slow the car down, and result in less power. 87 is the most volatile and explosive, but this same volatility will result in detonation being more prevalent, so the slower burn provides more protection against detonation.