It really makes a lot of sense if you keep the original cat-back section when you sell the vehicle. The new owner then has a very simple and effective means to return the vehicle to its original "silent" sound if he so chooses.

 

Cat backs on most modern cars are simply not necessary or worth the extra cost to the manufactueres due to offering no gains.

Moder cars, especially the e60 has a very efficient route from engine-headers-exhaust piping-muffler that there is no benefit to be had but replacing all that after-cat piping. The only true restrictive part of a modern car is the rear muffler as it is made for sound deadening. The piping leading up to the rear muffler is very straight, soft bends and a good size-so it is easier to offer replacment rear mufflers as that will offer as much performance as a cat-back in the majority of non-turbo modern cars.

Less work to install too-and offers all the same sound benefit.

 

Thanks for the response, Diamond. I wasn't thinking there were additional gains to be had through a cat-back option, I view this option as a "cleaner" approach to changing out our exhaust. No cutting and changing back to stock would be as easy as tossing the car on a lift.

It's my understanding that the bolts on the back end of the cats are easy to reach. I can only think that third parties don't want to take the time to prepare a proper two piece (necessary so it can be shipped) cat-back option either because they are lazy or don't want to spend the R&D dollars. Does anyone know the interior diameter of the cat-back exhaust prior to the muffler? Would the 550i benefit from larger diameter tubing?

I know a certain amount of back-pressure is necessary, but the third party vendors for the 350z community spent a lot of time researching these issues for optimal gains.