If you are getting P0420 and have an exhaust leak, you can be pretty confident that it's from extra air entering the exhaust system and triggering the code. I have a cracked v8 header that triggers a P0420, spaced the appropriate downstream o2 sensor and no more code.

The fact that you have power loss issues means you might actually have a plugged cat. Here's something to try. Take an IR temp gun and temp the various cats and see if any of them are far hotter than they should be. Also, look at your short term and long term fuel trims if you have OBD2 capability.

I think you're good here but its worth saying: NEVER space the upstream sensors. That's how the engine determines fueling in closed loop (part throttle and idle essentially). Only the downstream sensors should be spaced to defeat the P0420 in the case of an exhaust leak triggering the sensor.