Hey mate.
Only selling them as I will be fitting a Berk CBE in their place. The decats are great, but I ideally want a lower tone to the system without making it too loud. For some, running the ART pipes with a CBE will give the lower tone and volume. I just don't want to have a loud system, so running only a CBE exhaust should give me the tone I want.
The ART Pipes are great, and as above, they are suggested to give very good gains compared to other HFC and test pipes. For me it is purely about getting a good sound to the car, without too much volume. I know some folks run both and have no complaints, but I don't want to risk it.