Looks like the side exhausts are an extension of, or provide the
same benefit of, an H pipe.
I suppose on an E body the side exhaust would have to dump
right in front of the rear tires.
Probably increases power output, by allowing less
back-presure after the exhaust manifolds.
4 pipes are gonna flow more air than two pipes.
So in a way, they are muffled cutouts.
The stock AAR side exhausts are known to be very restrictive,
about 175 CFM each side.