You cannot over simplify the exhaust system on any mass produced car to that extent. All exhausts are a compromise and a balancing act between flow, noise, what is good for the engine and how much it costs to produce. The latter is becoming increasingly important these days as the number of serious recalls due to using "cheap" parts in the initial build demonstrates.
BMW, along with every other major motor manufacturer, build cars to make a profit which they do by selling as many as possible in as many different markets as possible.
By consequence, they have to satisfy many differing legislative requirements whilst appealing to everyone from "boy racer" to "little old lady" no stereotyping or sexism intended.
You might be prepared to put up with racing volume exhaust notes, rock hard suspension that is so low you cannot get over speed ramps, carlos fandango wheels or matt black bonnets but there are many content BMW owners who would not.
BMW produce a car that is competent in all areas and good in others that appeals to the widest audience but if you want to buy one and change parts being prepared to live with the side effects that come with those changes that is up to you.
If you want a bespoke car from the factory you might need to chat with someone like Bentley or Aston Martin but their cars will make a BMW performance exhaust look like loose change in your pocket!
