Would it make any difference if I ran higher octane in my Miata?

Miata's are supposed to be extremely good on gas mileage. But my city has nothing but Ethanol mixed in with their gas bringing down my mileage from its original 21 city and 31 highway, down to 19 overall, including highway. Would it make any difference if I switched to a higher octane? Should it be 87 or 93?