Misfires and low power

Had the CEL and “slow down” light up in my 2004 360 Spider and the car lost a lot of power. Exhaust smells rich. Idle is a little low, maybe about 900 RPM's, and the car has almost no torque/pick-up when the accelerator is pressed, very slow to pull away.

The OBD says misfires in all left bank cylinders (8,6,5,7), and O2 sensor malfunction.

My mechanic cleaned the MAF’s, replaced the O2 sensors, and checked for an air leak but could find none.

The left cat glows red.

Our current theory is a crack/leak in left exhaust manifold causing the O2 sensor to send a too lean signal to left bank ECU which in turn produces an over rich mixture leading to the misfires and "glowing red" left cat. Thermo couple senses temp over limit and produces "slow down" flash to annunciator. Planning to smoke test the headers on Tueday.

Does this make sense? Please let me know if you have any other theories. Thanks.