Installing anti-fouler fixes to correct a P0420 code can have unintended consequences, such as the loss of low-end torque that you have experienced. The down-stream oxygen (O2) sensor can have
an impact on fuel management and air/fuel ratios, which could contribute to the loss of power. It's possible that the O2 sensor is now spaced too far away due to the anti-fouler fix, which could be affecting its readings. I would recommend having a professional diagnose the issue and determine the best course of action to avoid any potential damage to your engine.