mine pulls off 89-90db. its borderline but its a pass, the VIC system where it has to be below 91 from idle to test revs and back it wouldnt pass. there is just no way to get a large pipe legal at low revs there isnt enough flow for the mufflers to work properly even my massive mufflers only manage 92-93db at idle.
on the back its a 18"x11"x8" offset -> centre 3.5" straight through and a 10"x8"x5" centre > offset 3.5" straight through in the middle.
the more gas you pump through it the quieter the system gets, there is more road/wind/engine noise cruising around at 3000rpm than there is exhaust which is fine by me.
when i was looking around i was asking about custom made to order muffler sizes it adds a bit to the cost as the mufflers are $250-400 each depending on size and material (stainless vs mild steel) and then you still have pipe + flange + labour costs. the way i look at it the bigger the muffler you can stuff into the masses of space on the back the more ground clearance you can have from the middle one. i really wouldnt be going much more than 5" high in the middle as thats pushing it though.