All the advice I've seen is that the middle silencer does most of the work to cut out the "boom" at 2-3,000 RPM. Which you'll want if you are doing significant miles.
Currently, I have the large Ragazzon silencer with unsilenced quads, everything else standard.
It's fairly loud, and probably a bit too unrefined for my liking. As for the boom, it is quite noticeable at 2,500 RPM.. much quieter in top gear at 70MPH rather than 60MPH for example (where speed limits allow..)
But it sounds nice at idle with a deep sound.