The octane rating only indicates how much compression the fuel can take before combusting without a spark. Higher octane fuels do take longer to burn, also. The higher the compression engine you have, the higher octane you should run. The same is true of advancing your timing. It's kind of a combination of the two.
If your car runs better and doesn't ping when under load with the 87, you may as well save some money and stick with it. Running too high of an octane fuel can actually lead to fouled plugs and carbon build up if your car isn't tuned correctly for it.
The other thing you might try is to advance your timing more for use with the higher octane. This will give the fuel more time to burn and may give you some more power in the process.
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351W 89 Mustang GT Convertible