Noone really knows OP, until the stock fueling limitations are overcome, the bottom end hasn't really been put through its paces. There are hundreds, maybe thousands, of FBO N54s running at/around 400whp for years with no internal crank issues, and a good number of 500whp guys doing the same with RBs. The Shiv big single seems to be pushing the most power, but at that point it's the tranny i'd really be concerned about. I believe VAC had a post showing that earlier N54s had very well cast pistons with forged rods; the crank has shown itself to be pretty damn robust so far.
E88 N54 Alpinweiss/Coral Red/Motiv HTA 3586r Tial .82AR/Motiv Port Fuel/Meisterschaft Race/Berk Resonated/ER CP/Synapse/VRSF FMIC/Rob Beck PCV Valve + Cap/ST Coilovers/M3 FCA + Tension Rods/M3 Subframe Bushings/M3 FSB/AA Strutbrace/DINAN Camber Plates/Apex ARC-8/Project Kics/VAC Hubs/Rogue Transmission Mounts/Alpina TCU Flash/Icarbon/Kerscher/BMW Performance