imo..if you bought a car and they did not disclose engine modifications such as a software reflash, I would go back and demand money back in the tune of thousands.
contrary to having a car and modding it and feeling great, mods decrease the resale value of a car significantly since it is indicative of certain abuse and possible harm to the car.
Definitely go back and complain, its illegal to just toss it your way without any documentation of such modification which implicitly voids your BMW warranty on the engine.
get some ZMP out of it and some wheels
"Your first 10,000 photographs are your worst.” ― Henri Cartier-Bresson