Actually, the need for back pressure is a misconception of the theory. Back pressure is inherent
in properly designed exhausts systems, so people equate it to being a requirement. Back pressure occurs because exhaust gases move through in pulses or waves, a result of the valves opening and closing. Each high pressure wave, or pulse, has a corresponding low pressure volume right behind it, creating a push/pull scenario within the pipes.
The key to a good exhaust is exhaust gas velocity, and you want the smallest diameter pipe that will push gases out the fastest, with the least amount of back pressure. Case in point: a large diameter pipe = lessened or no back pressure, right? Right, but
that's not why you lose power. In that bigger volume, exhaust gases expand & begin to cool, which reduces the exit velocity. Now you can see in that example that it's not the lack of back pressure that screws things up, but rather, the internal volume of the pipe that actually slows down
the flow. Effects on the catalytic converter has more to do with air/fuel mixture than flow velocity or direction. Lean mixtures don't allow the cats to operate at the higher temps (500-750 degrees) they require to be efficient, and richer mixtures contaminate, overheat, and break down their internal chemistry