DJB3

My MY05 WRX has started throwing up CELs. It's been intermittent but OBD2 code reader says it's P0420 (catalytic converter below efficiency). Have discovered today that rear cat (the one with rear 02 sensor) has a leak (probably rust). Can't be bothered with MOT implications of full decat (not to mention the need for remap). Is it possible to replace rear cat with decat stainless steel section and not have CEL on? The one that I've looked at has threaded hole for 02 sensor.

D.