every engine is different. and yes in SOME engines, higher octane fuel will produce more power, while in SOME engines lower octane will actually produce more power. to say it is ONLY to avoid knock is wrong. there is alot more to the power stroke of an engine involved. the octane rating determines how long the fuel will burn. (slow and long, or fast and short) the key to getting the most power out of ANY engine, is to get the burn to last from TDC to BDC without going out of that range, as well as 546878 other factors.
for example, if it takes your piston 2 seconds to go from top to bottom, you want to get a hot flame that lasts slightly under 2 seconds, timed perfectly. if your fuel does not completely burn within 2 seconds you get a knock. on the other hand... if your octane is so high that it all burns in less then one second then you are wasting 'time in stroke' ...in theory... im no scientist and i dont claim to know all, but it seems obvious that to find the perfect octane for ANY vehicle, (if you are meticulous and its high enough priority to know if there is a small difference in power and/or efficiency) you would have to test different octanes on a dyno, with every other factor the same. the only people this would be practical for is someone that has access to a dyno and alot of time on they're hands. thats why vehicle manufacturers do it for you, and reccomend a specific octane when you buy a NEW car.
if they say it runs good on 87, theres probably a reason they said it. im done rambling. just using too many words to try to explain a detailed theory i have bouncing around in my head.... in short, use what works best in YOUR vehicle, suiting YOUR needs.
1986 Camaro Berlinetta (originally)-now a nitrous breathing 383 stroker
1992 Camaro RS - pretty much stock (only to keep its 25th anniversary edition book value)
all the other Camaros were sold