Well, Harleys are not really known for being high compression beasts, is it carbed or fuel injected. My carbureted Evo runs great on 87. I don't know how computerized the newer fuelies are.
I never buy higher octane than I need, just wastes money. Even the wife's turbo Volvo works great on 87, the computer adjusts things so it doesn't detonate, and we haven't noticed any loss in power.