I have a cheaper solution. Saito makes a 90 deg elbow for less than $15. It works pretty well, and doesn't seem to rob power with 1 extra joint. I have this setup on my H9 P40, and the muffler is completely hidden in the cowl.

I have used Performance Specialty muffler on my YS 4-strokes. They work well, and do not rob power like YS stock mufflers do.