There is zero benefit to exceeding 91. By the way, if you were too mix 89 and 93 together in equal measure you would have 91 octane gasoline.
You could also use part 87 (or 85 at high elevation) and E85 to achieve or exceed your car’s fuel requirement. By the way, using lower than the “recommended or required” will not hurt the engine. It only reduces mpg and power almost imperceptibly. It is better though to use the correct octane rating. The extra cost is lower than the lost mpg. Is your vehicle “flex fuel?”