Wow, that looks great! The welding work looks to be top notch, and the fitment seems to be great.
I wonder if we could use an o2 simulator (similar to what the 335i guys use when they remove their cats) and remove the catalytic converter. Based on your pictures, it seems there's just one o2 sensor after the cat. For example:
http://store.bimmerworld.com/oxygen-...tors-p871.aspx