87 through 91 are relatively close in terma of how much power can be extracted from an engine. for example, if we take a typical engine, say a honda b20 and tune it for max power on 87, and 89, and 91, the 3 will have very similar power requirements, even if the 89 and 91 engines have a tough higher compression.
were talking about only a couple horsepower here folks. im sure you all know that octane rating in and of its self does not increase power. its the ability of the engine to let you modify it to higher compression ratios, change the timings, etc that increases power.
another example, my old mazda protege. it has a facytory timing of 10btdc on 87. i ran it at 14 btdc and realised a slight mpg improvment and possibly a small power gain. honestly i didnt care about power but others had dyno tested the timing change and noticed about 5hp gain. other people with this engine had tried premium and mid grade and the highest anyone was able to generally get out of it on pump gas was about 18btdc. but at that point they were actually losing power from overly advanced ignition. although it wouldnt ping on 18btdc and 91 octane, it didnt make as much power or have as good fuel economy as 14btcd, reguardless of the fuels octane rating.