I finally made good on my threat to try the genuine Robin side mount muffler kit. It took some creative plumbing, but I got it done. I hope to post the pics someday (probably some foul-weather weekend).
It doesn't live up to my hopes. The quality seems fine, but I didn't gain enough in performance or sound control to make it worthwhile. It is somewhat quieter and more of a mellow tone, but it's still not quiet by any stretch of the imagination. The configuration of the manifold doesn't come close to lining up with the cutout in the engine cover, so I had to add several elbows, which may have hurt flow. There may be a slight power increase, but I'm not sure. It still backfires.
If I ever try again, I will probably make a manifold out of butt-weld pipe fittings and use a much larger-volume quiet muffler, possibly from a small car.
Dave