Thank you Texas, that is the resonator that I removed. And Blklacker, out of the 4 cats only the foreward two have o2 sensors, so technically the 2 cats located further aft can be removed w/o a throwing any codes or cel's
a quick update: after driving with the resonator removed for a few weeks now, I would not advise it as there is more resonance (who could have guessed) and a slightly tinier raspy sound, it is hard to notice, but especially prominent when the motor is cold and the A/C is running. I am currently on a short vacation, but will be re-installing the resonator when I get home.
Debaffling the muffler: I have done some research on how muffler's work and have found that there are (correct me if im wrong) two main types of mufflers one uses glass (fibers) as insulation and the other uses a series of chambers (usually two). I am worried that BMW employs the later, the issue being that there simply aren't any baffles to cut out if this is the case. Thats because this design uses an entry hole and a resonation chamber to cancel sound before the sound waves exit through the exhaust pipe (tips). The reason the sound cancelation works is due to the size of the chamber which is precisely engineered to cancel noise by bouncing incoming waves off a wall and inversely matching the incoming wavelength. In theory, in order for it to work incoming trough's much match outgoing crests precisely.
If you have no Idea what I'm trying to say check here
http://auto.howstuffworks.com/muffler4.htm
Anyways: I may be putting this experiment on hold, until I have the funds to buy an aftermarket system outright if I screw things up, Unless someone can enlighten me on the inards of the stock muffler then I would be happy to try it.