We'll assume for the moment that dyno numbers are accurate. There are two aspects to consider regarding flow. The first is the simple restriction of flow due to the diameter of the exhaust, or construction of the muffler/cat. This would be the electrical equivalent of DC resistance. The other factor would be resistance as a function of RPM (frequency), and is the electrical equivalant of AC impedance.
The two exhaust systems would have to be characterized for both aspects. Simple resistance would be measured on a flow bench. For the impedance an acoustic tranducer, impedance analyzer and a tone generator would be needed, and measurements would have to be made from 0 - 1125 hz (3rd harmonic of 7500 RPM).
For an engine that's making over 75 hp/l, you have to figure that the factory engineers had a good idea of what they were doing.
