Before you fit something to your car, you should always find out what it actually does.
Put very simply, all the air going into your engine is measured by an airflow meter and the ECU then decides how much fuel to put in. An atmospheric dump valve vents some of that air out of the system that the ECU still thinks is in there. This is what makes the whoosh noise. So your ECU still thinks that air in inside and adds enough fuel to suit, thus momentarily overfuelling. Therefore, every time you let off the throttle, the dump valve goes whoosh, your ECU goes ???? and it can make the car run slightly lumpier than it would normally.
A custom map is a map specific to your cars and its individual mods. A generic off the shelf map is not specific to your car and therefore not as good. It's like the difference between and "off the shelf" suit and a tailored one.