My little spy heh Giggles, A/Gas coupe.
A simple question got a simple answer.
I last ran the headers out the sides in my XP Ute as did Haddie in the Commodore, Bob Ellis in his coupe and others.
I've since came to the conclusion that it was a mistake as it screwed up the airflow beside the body.
Big Gaz does it because of lack of options/space.
Ideally from my study with what we do, (and lacking Renaults wind tunnel) we should dump the exhaust into the void of turbualant air behind the vehicle to help balance this area some.
This creates a catch 22 for me as an engine man because a well tuned race engine has a very exact header length, and the style of cams we choose are often effected by any sort of exhaust.
So again as an engine man and remembering the car is merely a life support system for an engine the engine always wins with me. Correct length headers that exit under the car!
Form your own opinions. Results may vary.