@kennman
The Performance muffler box is pretty much the same size as the standard muffler. It is inside the box/muffler that is different - there is less sound deadening material and less baffles(I think).
Why not order a Performance muffler from a 120i and see how that sounds? You might also want to delete your middle muffler - in the mid pipe.
120d exhaust
http://www.realoem.com/bmw/showparts...61&hg=18&fg=20
120i performance muffler...
http://www.realoem.com/bmw/showparts...82&hg=18&fg=95