Al you're preaching to the choir here haha. You say this a lot and I say this back, you're taking octane rating in isolation of other factors such as the composition of the fuels in question. In this case the 87 contains 10% ethanol (YUK!) and the 91 is a "pure" gas. Octane numbers here are the pump numbers of course R+M/2.I’ve been preaching this for 20 years. High compression engines need high octane fuel, in most cases, stock kart engines are not high compression.
Octane is not a measure of power, it’s a measure of how good it can resist detonation. The higher the compression, the more likely high octane is going to be better. It’s the compression that gives you the power, not the octane.