earlwb

One thing worth noting is that the reverse cone on the muffler/tuned pipe works best if it is a straight tapered cone. A curved cone doesn't work as well. Less of the shock wave gets reflected back towards the engine when the cone is curved. Thus the short fat muffler chamber using a curved cone is not as good as the long skinny muffler with the straight tapered cone. When you look at the tuned pipes and expansion chambers they all use a straight tapered cone shape.

But anytime a muffler lets the engine develop more power than just using a open exhaust makes that design successful. So both of your designs are good.