If you run the car in a racing environment, the added octane will only help if A) The car is built and programmed to take advantage of the high-octane fuel, in otherwords, if it can adjust the timing to take advantage of 103 octane racing fuel, or if it's computer will not adjust it beyond a certain point. If the computer will only adjust the timing to performance levels that are covered by using 91 octane, then it don't matter how high above 91 you go.
B) If you're car is running in a hard, hot environment, like a road course or drag strip, and you are experiencing knock and other related issues. Some cars are better than others at dealing with track conditions, and some will benefit from higher octane fuel in these situations.
Car and driver did an whole test section in which they took cars that required various levels of octane, 87, 89, 91, 93, and ran them on a dyno with various levels of octane in the gas. One of the BMW's in particular, actually put out less horsepower with 93 octane than with 89 octane. Don't quite remember what it was. None of the other cars did this, nor did any of the cars that call for lower octane fuel generate more horsepower on higher than recommeded octane levels. The cars that required 91 or 93, did lose horsepower, a great deal in some cases, by running fuel that was below the octane level recomended.
Bottom line, read the manual. It says 91 or better octane required.