I'm currently running KP primary/secondary combo, never had CEL after the install since I used a set of these
O2 defoulers.
Since you've already had the downpipes installed, to avoid a trip back to the dealer you can easily install these yourself. Have a look at
this thread to get a closer look at where they're supposed to go (2nd last pic).
Basically just unscrew both your post cat sensors, plug these in and install the sensors back on top. This can be done from the engine bay with the car on the ground, it's not a guaranteed fix but it did work for me and many others. Hope this helps and good luck!