[750hp]

I've had cars with rear exiting and with side pipes. My personal preference is for live side pipes, but that's not the question...
If the issue is purely around the sound and if the decibel meter is positioned a specific distance from the end of the exhaust pipe (regardless of where that might be), why should side pipes necessarily be louder than rear exiting pipes?
You can fit over a metre of muffler material into the side pipe, so why should that not be as effective as one or two regular mufflers?
If the issue is a lack of a balance pipe between the side pipes, that can and has been incorporated into some Cobra builds.