Back in the day of carbs and distributors we used to remove the silencer for racing and substitute a straight through pipe (sometimes with a baked bean can painted black as "visual" silencer)
This certainly gave more power (backed up by dyno figures) but more importantly it enabled you to hear the engine note of your own car in a field of very noisy similar cars.
Now that ECU controlled fuelling and ignition is pretty much standard, the optimisation of the exhaust is best done scientifically. Also important for road cars is the effect on emissions. It is no good having a great system if it no longer allows the car to pass the required figures at MoT (or equivalent) time. Euro 5 emissions standards are incredibly tight, and any changes to the exhaust manifold and position of the cats is going to have a major effect.
