Even Pud(Dan from Autolusso) has now fitted a silencer to his Wizard quad system, presumably because that too was excessively loud.
We use the same backbox on all our systems, but the volume can differ from one to another for lots of different reasons. Main difference is how they sound on a CF2 compared to a CF3. Subtle, but noticeable difference in pitch & tone, but similar volume provided all is standard. I like to think the single, big CF2 silencer gives similar silencing to the 4 small ones on a CF3. Which is why a CF3 with front manifold cats removed ends up being louder than a CF2.
On the 156 GTA we used unsilenced centre sections and also the rear box does not have any packing material in it - it is chambered internally to allow resonance to cancel out sound waves of certain pitches and enhance the sound waves of others.
On the GTV V6 it turned out that the unsilenced system was a bit louder on idle and full throttle than the 156GTA/147GTA, even with the same backbox and the same type of unsilenced centre section. The reason for this is because of the difference in length between the exits from the backbox and the ends of the tailpipes. On the 156/147 GTAs they are about 9" long or so - on the GTV they are nearer two and a half foot. This causes extra resonance and its where the extra volume comes from - although when warm its roughly on a par with the GTAs, when cold it is louder.
Now it has a small silencer fitted it sounds much quieter on idle, which was the only real criticism you could give the system. If we could have located the silencer in the middle of the rear bumper with much shorter tailpipes we could have done without it, but the silencer design wouldn't allow it, the plumbing forced us to keep it 'longitudinal' compared to the original 'transverse' silencers on the standard system.