It seems to me that taking out all the packing and swapping perforated pipes for solid ones, functionally leaves you with a muffler delete that looks like stock mufflers.
AFAIK, there is no difference in restriction between a perforated straight pipe and a solid one. Think "Cherry Bomb".
Not certain about the Helmholtz resonators, but I doubt they create much restriction. Notice Dinan is using them in their exhaust.
I would definitely like to hear more exhaust sound from my 335. But I'd also like some increase in flow. And yes, a stock appearance would be desirable in PRC (People's Republic of California). If you can get the performance without drawing too much attention then all the better.
This makes me think about the 10 or so different exhaust configurations that I tried on my first generation Mazda RX-7. At one point I had headers, two straight pipe mufflers in tandem, and a Supertrapp with 18 discs. Basically straight exhaust with 3 perforated pipes and a "sound diffuser" at the outlet. Full throttle through a highway underpass sounded like a swarm of killer bees was chasing the car down.
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