To gain power you would need to modify your tune because because the car won't detect the higher octane and advance the timing for you. The other reason to run higher octane is simply as insurance, to give you an extra safety margin, particularly if your tune is pretty aggressive. The thing you have to be aware of is that it is also possible to "over-octane" a car--i.e., higher octane fuel burns slower, and if it's way too high for your particular combination, you will lose power by slowing down the burn rate. If your car runs well on 93 or 94, 100 might give you a little extra safety margin without causing any problems, but I'd stay away from anything higher than that unless you're going to modify your tune. I don't know how nitrous affects the situation.