93(E10) vs 91(E0) will definitely have a difference. Ethanol is a GOOD thing, not bad. Your 91 is not only going to have less power potential vs the 93, just because of Octane, but 10% Ethanol makes a difference too.
I would not be willing to guess a difference, but even on Stock tuning you would notice HP difference, especially on a long pull, or heat soaked between 91(E0) and 93(E10).
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