Sluggish acceleration

Engine has had new head gasket replaced and head checked. Engine starts and idles fine but is fairly sluggish on acceleration. New plugs (less than 10 miles) show lots of carbon on the back cylinder bank and pretty normal, light colored on the front bank. Catalytic converter is more than half plugged. EGR looks good, new cap and rotor, coil pack tested with ohm meter. Back oxygen sensor is black and carbony, front one is whitish(most likely from burnt engine coolant due to head gasket). Would o2 sensor in rear bank cause such rich conditions and power loss?