My comment was a generality. It's significantly easier to run 2 smaller diameter exhaust pipes than a large single and you can get plenty of flow over a large single. Quoting the actual science:Untrue.
Cross section of a 2.5" pipe allows for 509cfm theoretical max flow. A 3.5" pipe allows for 1029cfm, for example.
Volume flow rate = π X pressure difference X pipe radius 4 X liquid viscosity / 8 X viscosity X pipe length.
F = πPr4 / 8nl
To put this relationship into words: At a given temperature, flow rate through a tube or pipe is inversely proportional to the length of the tube the viscosity of the liquid. Flow rate is directly proportional to the pressure gradient and the fourth power of the radius of the pipe.