[Ryland]

You are never going to see 0 pressure in the exhaust, pretty much the lowest you can get it is 3psi, back pressure is a lousy wording because it does not describe what is really happening, it's pulses, just like a musical note from a trumpet is sound pulses, toss some weird sized chunk of pipe in there and it makes it go out of tune.
An exhaust pipe is part of the engine, if it was just a duct for exhaust then it wouldn't matter, but if you pull the exhaust manifold off the head of an engine it will hardly run, but at that point it will have -0- back pressure, exhaust is a gas, gases fallow rules of fluid dynamics, it has weight and momentum and it moves in pulses.

Fix the rest of your broken system before replacing or destroying an expensive part like a catalytic converter, the cat might not even be broken or clogged if this is a new problem.