I would think that since the restriction is effectively reduced, then there would be a fairly smaller noise level. I'll use this example.
GT500 with Kenne Belle blower and stock headers hitting 10 psi. Improved header flow by increasing primary diameters. Psi on same pulley drops to 7. So effectively not working as hard for equivalent to more power. It was more power. That would tend to make me believe that the noise level would drop a bit, however, the exaust primaries being bigger might make it stay the same, if not get a little louder. Was done on an engine dyno, so the there was no cat back.
Either way, I'll say a little quieter.
