After much research/reading, I finally came to a conclusion.
- All 335i X-drive E90/E92 (both N54 & N55) have secondary cats
- 335 N55 E92/E90 (RWD) have no secondary cats
- Only N54 E90s have the resonator on the rear section of the exhaust (from the secondary cats, back)
- 335 (X-drive & RWD) N55 replace this resonator with the X-pipe (also on N54 E92).
So, the "midpipe"/X-pipe/Y-pipe that comes with the E90 N54 BMW Performance exhaust eliminates only the resonator (it does not eliminate the secondary cats). Therefore, this is of no benefit to any E92 or N55, as they already have this pipe from the factory.
Hope this helps someone
_____Stock N54 E90 Exhaust with secondary cats + resonator_____
_____X-Pipe that only comes with E90 N54 Performance Exhaust Kit_____
_____Stock N55 E90 X-drive Exhaust: Secondary cats, with X-pipe, No Resonator_____
_____Stock N55 E92 Exhaust: Catless, no resonator, X-pipe_____
