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...?
2009 335i 6MT
(E92/N54), Black Sapphire / Shadowline (Individual 760) / Black Dakota Leather, ZSP, ZCW, 6FL, PDC, no Nav/iDrive, 19 x 8.5 / 19 x 9.5 VMR VB3 in Hyper Black, Pilot Super Sport 235/35 fr 265/30 rr, M3 control arms + tension links (fr)