Octane requirement is a hardware thing. If the engine needs 91, then you have to use 91 to make full use of the power it can produce.
But if it only needs 87, and you think 91 will give you more performance, then you need your head checked :-)
That engine will not be able to make use of 91 regardless. Unless it is equipped with dual sensors for 87 and 91 in which case it will optimize power for whatever fuel it detects without and loss as opposed to using the wrog fuel for the wrong engine.