It's not just raw flow capacity that determines the performance benefit (or drawback) of an exhaust system. Gas velocity through the system is very crucial. Faster velocity means better scavenging of the cylinders, which means better performance Bigger-diameter pipe reduces velocity, which reduces scavenging, which lowers performance. You want to come as close as possible to the optimal flow capacity for any given engine configuration; extra flow capacity is not better, it's worse.

That's just one example of why throwing together exhaust systems or trying to make headers without knowing what the H-E-double-hockeysticks you're doing is a waste of time, money, metal, and weld rod.

That's why I stated "so long as you have the appropriate diameter (both for the headers and the rest of the exhaust)." But what I was trying to state is that logic tells me dual exhaust and single exhaust should have no performance difference so long as the surface area of the the two dual exhaust lines are Identical to the surface area of a single exhaust line, and you make use of an X-Pipe on the dual exhaust system.