You don't have to be an exhaust guru to understand the advantage of a performance single exhaust on an inline engine. Look at this picture.
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The firing order on over 90% of inline sixes is 1-5-3-6-2-4. Now, separate the tubes into two groups of three. It becomes easy to see that with the fire of every cylinder, that it swaps from one group of three, to the second group. With a wye at the collectors going two into one large pipe, you can now understand the effect it would have on scavenging. Each time a cylinder is fired, it causes a low pressure area in the side that has not fired and helps pull more exhaust out. Every bit of that scavenging is lost going to dual exhaust. The slant 6 probably needs that more than any other inline engine, because of the poor cylinder head design in comparison.
Duals can certainly sound good, but I would be willing to bet a good performance single exhaust will make more power and torque, all things equal every time.