Octane rating has nothing to do with unburned fuel or carbon deposits. Also that test showing 5hp change does not tell the whole story. Since we see maximum boost down near our torque peak, that is where we are going to feel the difference the most. Higher octane gives you maximum power and efficient operation. If you drive for economy the efficiency pretty much offsets the cost premium and that means whenever you use it, that extra torque and horse power is essentially free.