When Motormech built my 3" system for Ten of the Best, it had no centre silencer - it made a lovely noise, but it was boomy at motorway speeds
I got Paul to fit the largest centre silencer he could find (its two feet long!) which improved matters a lot
There's still a resonance point at just over 2,000rpm, but its not bad and it doesn't annoy me, as motorway cruising is a few hundred rpm higher than this
It didn't change the performance, as its a straight-through silencer, with a perforated pipe and wadding to absorb the sound