One way to quantify that would be to weld a small pipe thread bung in the head pipe, just before the cat. You could then attach a pressure guage (0-15psi) to the bung and run the engine through the rpm range, noting the pressures at specific points.
You could then repeat the process after removing the cat and compare the difference. Idle pressure should be around 1.5psi (or less), 2000 rpm should be about 3psi and 4000 rpm could be anywhere between 4psi and 8psi. If the pressure shoots up to an abnormal reading, there is excessive back-pressure in the system, especially if it continues to climb while holding a specific rpm.
So far, I haven't actually tried this process. Like Cliff Clavin, this is strictly theory.
