You will only make more power with higher octane if the motor is tuned to take advantage of it...i.e. higher boost and increased timing. Same for water/meth injection.
"Silencing knock" is a misperception. Higher octane and meth works by lowering cylinder combustion temps which, in turn, decreases the chance of detonation at the same compression. This is what allows you to increase boost/timing back to the detonation threshold point. Increased boost/timing is where the increased power is coming from, not from the "race gas" per say.
This kind of tuning should never be done on the street. On the dyno with a set of "ears" to listen where the detonation threshold occurs...that is now your limit. The street tune is done after, staying below that limit. This is pretty advanced for most guys...increasing octane or using meth and then cranking the boost is asking to blow the motor. The 2nd danger is tuning for race gas then using pump gas on the street...your safety margin is radically decreased, resulting in a very likely change of detonation under boost. All it takes is a little bit of a lead foot and BOOM.
As stated earlier, using high octane gas on a motor a the stock ECU will result in lower power output. High octane gas contains less BTUs than pump gas.
Of course, all this depends on what EMS one is running