I would leave it be... but you'll have more luck over on the UK forum, sure I remember someone changing theirs. Speedreligion (now gone) used to do one and the manufacturer where they got them from was posted somewhere over there... I'll have a look.
ETA: Not found much but check out the following quote from the greatly knowledgable E92Fan... this is for a European car, so not sure if the urea injection would interfere with a replacement DP back exhaust (FYI: DPF is the diesel particulate filter found in the downpipes)
Originally Posted by E92FAN
I wouldn't touch the DPF at all - yes, it increase back pressure on the turbos, but the benefits it gives vastly outweighs the downside. The amount of soot that will come out of the back of your car without the DPF will be unimaginable. Plus you'll almost certainly fail any MOT in the future.
Far better to replace the exhaust post downpipes to free up the residual back pressure. And Jules is right - 350bhp is the MAX you really want to consider. Getting to 370bhp will require much more than a remap - the fuel rail pressure will be so high to get the right balance that it will be touch and go whether it can withstand the flow rate. Also you'll then have issues with cooling and longterm reliability. Minimum you would need a new oil cooler, uprated intercooler etc etc... Whilst the majority of parts can be taken from the basis of a 335i, I'm not sure whether you want to be the guinea pig for that...!