As you can see below, the exhaust found on the US spec 335d (2nd pic) and its' European counterpart are different.
The US spec 335d has a cat (1) to reduce the NOx, which also has a NOx sensor before and aft which triggers the DEF system. We understand what we need to do to get more sound out of our exhaust, but our challenge here in the US is the issue of reprogramming the ECU to accomodate the DPF removal. There are tuning companies such as Ecotune, but it would require removing the DME and shipping it across the pond to them in the UK - not very convenient, esp if when the DME receives a software update at the dealer.
The exhaust noise supression (thanks to the DPF and cat) is so bad in fact that you can cut the exhaust off at the mid cat effectively removing the mufflers and the entire back section of the exhaust and it makes little difference in sound.