From Sands interview:
Is the engine so highly tuned that any modifications will not help?*
Yes, to a certain extent. For example, Lotus will offer three levels of exhaust, each progressively lighter and louder. They cannot ship the louder exhaust on the car as the car must meet strict sound regulations, especially in Europe. However, once the car is delivered, and depending on the state, the owner can fit a different exhaust.
It will be extremely difficult for after market companies to offer an exhaust with the same performance level as the one offered by Lotus. The exhaust tube is bifurcated at a specific distance from the header. This distance is critical and if not duplicated, power will be reduced.
Lightest and loudest sounds good to me.