Getting ready to do this mod. I bought a full Milltek exhaust for my Z4 a few months ago and it did not fit properly, so I returned it. However, the cat delete pipes fit perfectly and they originally sent me 2 of the same pipe so I still have the right side pipe left over. I am going to buy the left pipe and delete the cats with this setup. I also am running AR DP's that have deleted the primary cats so I will be running a catless exhaust system.
The catless system with the Milltek mufflers was too loud for my taste so I am hoping that it will be perfect with the OEM mufflers.
I am curious as to what you will do when you sell your z4 and put it back to stock. Do you plan on welding the catted pipes back to the stock mufflers, use a band clamp or something else.