Johnny, I see your logic.
I think that be it a smaller pipe or the honeycomb in the cat a "bottlenck" can refer to any restriction that reduces gas flow the most in the exhuast system. After all if we add up the area in the cat's honeycomb that is "open" to allow gas through, it will equate to a pipe diameter. If that diameter is less than the other pipe diameters in the system then it is a bottleneck.
Maybe we should try an experiment. Say we get a large straw, we crimp a segment of the straw 3/4 closed, then a bit further down the straw we crimp it 1/2 closed. If we blow through the straw we will get airflow that is less than if there were no crimps in the straw, right? Now if we release the crimp that is 1/2 closed and blow will we get more air flow? Try it I bet you won't. This is becuase the 3/4 crimp is the bottleneck in the system. If we released the 3/4 crimp and kept teh 1/2 crimp then there woudl be a bit more air flowing through the straw...
This is why I don't believe that the restrictions are cumulative.
If there is power gains then it must be explained by, not flow rate, but perhaps flow characteristics... who knows?
