Sorry, but an exhaust system will improve even a factory non-modified engine.
Most manufacturers use mass producers to make them.
Most exotics are still the same way.
Let's look at what an exhaust does.
An exhaust is designed to help extract the gases out of the cylinders(scavenge effect). Normally aspirated engines have a period of overlap, when both intake and exhaust valves are open(critical period). This difference of pressures causes it to try to suck in gases that have already been fired.
Headers: Aftermarket headers will have a larger diameter and will help reduce the scavenge effect during the critical period, which will result in more gases flowing out, resulting in more horsepower.
Catalytic converters: Aftermarket ones are made for high performance, which results in better flow, giving more horsepower.
Cat-back: Again,larger diameter pipes equals better flow, aftermarket mufflers are specifically designed for better flow, also equals more power.
Koenig has a cat-back system for the Enzo, so even Ferrari factory systems have room for improvements.
And cats available next month.
K&N filter will help improve flow into the engine.
On my vipers**cough*cough** an exhaust system(headers, cats, cat-back) is good for about 50-55 horsepower, filters(conical K&N's) another 10.
Granted, it will not be as high on an exotic, but an improvement, nontheless.
Lambo's gain in the 30's.
BTW If you don't want it loud(race exhaust) they make quieter ones.