Im sorry but where do you obtain youre mis guided Info from , Headers will open the restricted airflo there for It will give you more sound , Secondly EBAY headers can be a bit of a bitch because you loose some low end torque . Ps not to mention youre 02 sensors will go off very so often thats If you add some 02 sims youre problems will be headache free.
Youre probably saying well jeez Paolo , Active swears bye It , Did Active also tell you (IF SPOKEN TO THEM) That having there bolt ons makes all the diff ?If youre not rocking any ACTIVE bolt on hence Full catback system and all the other whistles and bells Its not worth It.
What you really want Is to make nice with some guy whos been working on BMWS his hole life , What you could yeild some hp out of is getting it tuned(not a generic tune , a "individual" tune)
An individual tune optimizes the engines set ignition/fuel maps so that they work the best with your components / fuel.(the higher octane fuel you use the more advanced your ignition can be , thus increasing the tourque and hp of the engine.)
Optimizes in fueling will make your car burn the fuel more efficient(the stock dme tends to run the car a little rich, which can be leaned out for more power)
Being said 40 hp will never happen the most youll gain from these bolt on mods will be a mere 15 hp , If youre lucky , If you want real gains FI Is the way to go.
Sorry If I sound like an ass but Ive read countless threads In regauards to ECU"S REMAPPING EBAY HEADERS and all of the above , Im not saying that you wont feel youre car chirp but you wont begaining as much as every one states.
Call these guys (link provided below) and ask them any question you want , Im sure theyll tell you we can do this and that and bla bla , Ill belive It when I see actuall dyno sheets.
http://www.rpmmotorsport.net/