it been put there to filter out all the hazardous gases before release to air. some say got improvement. i never test it out. i just replace mine to after market. get better FC than original cat. as i know all car using ecu advised to set back the ecu for better performance.otherwise just wait the ECU auto learn your new upgarde. please do correct me if i was wrong.Hi, any idea if I take out the cat exhaust system from the original setting would it effect the ecu? Any different on the performance or the sound? Or can I replace to those after market mid box.