I am not tuned so I cannot speak from personal experience, but from research I've done in the past, I am under the impression you will only see gains with a catless downpipe if you were running a tune that has a map specifically programmed take advantage of the reduced restriction and increase boost beyond that of stock or the PPK1.
IIRC, the PPK1 will not detect that you have lower restriction due to the catless downpipe, and thus, will not increase boost netting you no performance gains. Once again, this is based on research I have done on the topic; if someone has first hand experience with this please chime in
I think you would still gain a deeper exhaust tone with the catless downpipe if you were to install it, however I have been told by numerous people not to run one without an aftermarket tune that is using a map specifically created for using a catless downpipe.
Hope that helps!