One more thing to point out - the xdrive 35i's have 2 midpipe catalytic converters. Removing those will take 20min at a shop, cost $30-50), and will increase sound and give you a 10-12whp gain, while NOT triggering a CEL. Plus it is totally reversible - just weld them in for another $30-50 if you ever want to go back to stock.
This is exactly what all the 335 xdrive guys did - the xdrive has the midcats (as do all n54 engines) and a popular mod is removing them (or replacing them with a N55 midpipe without midcats from a 135i, in the case of the n54.)
Also, the other thing I realized is that the BMW PE piping is the same size as the oem piping. Even if the entire BMW PE piping doesn't fit, there's no reason we can't get 100% of the effect of the PE by deleting the resonator, chopping the BMW PE muffler off from the piping, and welding back on to the stock piping.
The bottom line is that this CAN be done for probably $150 worst case installation at a decent shop. Doing the entire mod including secondary cat deletes should be good for around 15whp least based on other xdrive dyno charts (in addition to drivetrain losses, this is one of the reasons xdrive cars always dyno lower stock).
Last edited by paradoxical3; 03-25-2013 at 05:43 PM.