Actually, anything below 14.7:1 is rich. It's not VERY rich at 14.5:1, but it IS rich. Ideally, the car will be fastest at 14.7:1 at a perfect stoichiometric ratio. The chance of something small making the car go lean is why you don't want to be there at 14.5, but that's not lean. You're not feeling fuel. You're feeling the surge of the combination of boost and fuel, as there's more air in each cylinder and then the ECU adds fuel to try to match the air.
The reason you're getting more power is because the amount of fuel combusted with the increase in air is actually like having a larger motor. It all comes down to "volumetric efficiency." You're moving a greater quantity of air, adding fuel, and combining that with a controlled spark. You have to have all 3 to have fire and therefore, power.