I would completely disagree with what Speedwagon says. If the ecu is programmed to see 87, assuming you have not modified the car significantly via bigger cams, increased compression or some form of forced induction, the higher octane fuel will do absolutely nothing for you performance wise.
All modern gas has detergents in it (more marketing hype, sorry to say). If you believe that the big oil companies don't stretch the truth, then your being way to kind to them. Sure false advertising is illegal, but they are not really telling you anything except it has detergents in it. Nowhere do they ever say that their 93 octane has more than their 87 or 89 octane.just that the 93 has it..big deal.
What WAFlowers said is right on the money...its trail and error. My Legacy loves Shell and Mobil, and is not so fond of Amoco..why? Who the hell knows...my ZX which has around 600 HP runs just great on Mobil 93 for the street, and I have a tank of Amoco 93 in it right now and the datalogs all look the same.
If it runs better on the 89, thats cool....maybe now try some different brand 87's and see what it does.