The only way to definitively answer the question about the construction of the BMW Performance exhaust without cutting one open is to have one x-ray'ed.
Are there not large x-ray machines used to inspect aircraft parts for metal fatigue that could work for this.
Yesterday a 335is drive past me while I was waiting to make a left turn (and he's turning left from 90º to my right. He was accelerating quickly so I got a good chance to hear the is exhaust. Then I drove off and heard my own exhaust (stock 335) for comparison.
It seems to me that the sound of the is exhaust had a similar character to stock, except that it was louder and had more of the higher pitched noise.
So maybe it is the same design with less packing...?
For Sale: 2009 335i 6MT
(E92/N54), Sport / M-Technic / no iDrive / Black Sapphire / Black Dakota Leather, ZCW, 6FL, PDC, 19" VMR VB3, Pilot Super Sport, M3 control arms + tension links (fr) S. California