A couple options for the exhaust on the BS-7.
The stock setup, with the original chrome downpipe and muffler. It weighs 9 pounds, plus a pound for the rather robust mounting bracket, and is very much built for low-end performance. It also makes it harder to package the rear brake lever and footpeg.
Something I cobbled together over the weekend. It uses an old S90 muffler as the basis, cut into pieces and welded back up into the desired shape. It is built to give peak power a bit higher up (6,000rpm) in the range. It comes in at 4 pounds, and mounts to an existing hole in the frame. I plan on wrapping it in exhaust wrap, and building a small heat shield to keep my right leg from getting burned.
I prefer the high pipe, as it will make more power, and gives more clearance.