Hi Craig,
I think that just looking at flow rates for mufflers will not give you much indication on their performance on a given engine.
The most critical characteristic of an exhaust system are its pulse lengths. (Note: we will consider it a given that adequate flow area is available for the system)
The many variations in muffler design will give you wildly different pulse lengths, but will have comparable flow numbers.
The effects of these variations will give you very different power curves, mixture curves, and total HP numbers.
There is also the consideration of heat retention. The specific heat is very critical to the acoustic properties of a system. (IE: positive and negative pulse speed and pressures.)
For instance, the standard headers and sidepipes on the JBL are designed to produce maximum negative pulse strength at 4500 rpm on a 351.
By changing the secondary pipe (Sidepipe) pulse lengths, one can move the maximum negative pulse event up to about 7500. For anything higher than this, the whole system timing will have to be changed. (IE: primaries.)
In other words, exhaust design is rather complex and many factors need to be examined in addition to flow.
(Of course, all of this changes with a turbocharger. So anything that I said above does not apply to your car.
Except for specific heat.)