Running 105 octane on a stock car will not really get you anything, in fact, it will probably decrease power output since this fuel will burn much slower than the pump fuel your car was tuned for. Now, if you tune your car for 105 octane, you can probably make another 20-30 lb/ft of torque and even some more top end because you can run the car more aggressively with this fuel than you can with standard pump fuel.
Of course, custom tuning in addition to the cost of the fuel (which can be as much as $6 a gallon) is the price of running this fuel.
