Take a look at this section 2 for the M3, it is just like my center section (one 3" pipe) that I have a resonated aero turbine in. I have headers and the rest of the exhaust is straight pipe no cats, VERY little drone and great gas milage. Why not make something like this for the z4m I think it is a better option personally.
http://www.statusgruppe.com/products/e46-m3-x-pipe