Well all I know is, that ever since installing my CDA my car is definitely more responsive and also has more kick to it as well - I have the chance to compare whenever I want to my mums standard Astra :P

Also, speaking of modifications, engineers do a lot of things that can be improved upon. Exhausts are a prime example - yes the manufacturer puts effort into achieving the best balance of power and noise, but it is always a compromise, as they have to sell their cars to a large demographic. I guess there's arguments for and against both