Just going to put this out there ~ You can only really say that though if you had the car put on the dyno & tested firstly in standard form, ....then proceed to fit the aftermarket parts and re-test the power of the car on the dyno under the same conditions as before (E.g: same ambient temperatures, same day, same tyre pressures etc etc).
Also a car manufacturer may say on paper that their car has 276Bhp as new from the factory but the truth could be it's got more power as standard (used to be fairly common with Japanese cars from the 90's) or even less power.
Plus each dyno make will give different readouts as well as the same dyno make giving different readouts for the same car on different days. Again coming down to so many different variables.
Oh and just to put this in the mix as well, tests have shown that a car running on new engine oil as appose to 12 months or older engine oil will show an increase in Bhp.