high octane fuels usually improve performance, but it's a matter of how much. a few HPs? i'll save my money because you won't notice a few hp difference.
also, premium costs 20-40 cents more per gallon, that's almost a $5 difference at most. if the mpg difference is negligible, you're wasting money. the mpg difference would have to be enough to reduce the cost per mile of driving to sell me on it.
it would have to be +3mpgs using 91 for it to cost less compared to 87.