Sorry, but having a big chamber in the middle of your exhaust pipe is not good for scavaging.
It is good for causing lots of turbulence and slowing down the flow of exhaust gasses though.
The theory you are refering to is talking about the pre-cats being empty. (Again the "Mystery Mod" vs gutting the main cat)
If you must remove the cat. Put a patch piece of pipe in. Do not just gut it.
