Kind of wanted to get a side exhaust set up ('14 GT owner) and thought I had a handle on understanding the set up, but after reading; I'm not sure why the exhaust fires out the sides while still having axle backs out the rear. Is it a cut out situation...what's the point otherwise?
The Boss side exhaust has branches that let unmuffled exhaust to come out ahead of the rear wheel. There are restriction orifice plates to limit the flow out the side (otherwise the volume would be pretty loud). Most exhaust flow goes out the axlebacks. If you find a photo of the Boss exhaust you'll see the little side branches with a set of flanges where the orifice plate sits. The Boss exhaust has a unique sound.
The Cervini's is a true side exhaust without axleback mufflers. All of the exhaust is routed to the sides (ahead of the rear tires).