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if your trying to compare the efficiency of a normal cat (clean not clogged) compared to a hollowed out cat then from what i have seen there will be little to no difference.
I looked at this myself some time ago, took the cat off the car, i was going to hollow it out then looked into it and noticed i could see straight through the cat, there was a grid of metal to provide a large amount of surface area but it was constructed in a way to provide little to no impedance to air flow at that point i didn't bother hollowing it out.
If however your looking at the difference of a clogged cat compared to hollowed out then i am sure there will be a big difference as the engine works of a pressure differential between the pressure generated by the expanded gasses and the pressure of the exhaust.
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