Run 91 octane in your custom modified engine. Forget about ethanol problems, unless you store it for winter, then use a suitable gas stabilizer.
Don't bother with any additive. Use the gas from the pump that runs best.
Small utility engines...lawn mowers, snow blowers, generators, etc., are doing better on hi octane gas in some regions of the country. The regular gas has some compounds that cause intake valve stem deposits, and premium doesn't cause this problem, at least according to a Honda Power Equipment district rep.