The Cat still had a better range and better combat record. The Cat was better at U-boat sinking, which was the Sunderlands role.
That's a nice thought on the Roc but Luftwaffe pilots weren't tunnel sighted. And it would have been a very awkward manuver with the slow speed of the Roc, it would be there long after the Spitfires and Hurricanes, not an effective decoy.
I wouldn't like to be the bait though.