Based on specs, it really look like a detuned N55 (gas), like Audi did with the 3.0T found in the A4/A5 Sportback.
I have this crappy pic. I am now mad at myself not having examined it closely. But you know my interest in diesel. It may even be a 4-cyl., which doesn't explain everything.
2007 E90 AW 323i Step | Lowered 1.25"/1" | BMW Performance Exhaust | PBX | Debadged | Scangauge II | Style 162 18" & 161 17" wheels & rear 15mm spacers