I never actually measured/dyno'd the difference in a stock box vs a WAI/CAI really, but there should be nominal increases in performance and efficiency for the upgrade. Remember it's a simple plug & play upgrade you're not forcing anymore air into the motor just allowing the engine to breath more and better. Maintaining a clean/clear intake will create endurance of the performance. Remember, all cars have to have a balance when it comes to airflow... you can't force too much air in and the outlet doesn't remove the air quick enough which the car can run lean thus screwing up your fuel efficiency. Having a proper ECU tune with these common bolt-on's and an adequate exhaust should gain you the numbers they advertise.
Don't expect 6~10WHP out of the box. Just new noises and marginal increases.