This. If you're going cat-back a tune isn't needed. If you've got the downpipe and are going downpipe-back a tune is a mustThe exhaust isn't the real concern, running a different downpipe without a tune is pointless though. Yeah it might spool half a second quicker, but it still hits the same torque target and pulls boost out. Even worse it might just be messing with your fuel trims a bit and actually making the ECU compensate.