Congrats!! You have almost the same exact car I want! So jealous! :-)
So, if I'm not mistaken, the M Performance Exhaust comes with or without the carbon fiber "tips". To me the carbon fiber ones look to be "slip on". I'm personally on the fence as to if I would want them or not but I believe they are pretty expensive in addition to the exhaust (and you need 4). If you are a carbon fiber "freak" (no offense) then go for it BUT I beleive the rear diffuser you are talking about is also carbon fiber (I want it too):
http://www.shopbmwusa.com/PRODUCT/53...-REAR-DIFFUSOR
From the pict this is essentially what the rear diffuser would look like with the M Performance exhaust sans carbon fiber tips. Quite honestly IMHO a carbon fiber diffuser PLUS carbon fiber tips would just be too much carbon fiber in the rear.
Hope this helps!
...and yeah what's the M Performance Interior Package???