Your 17-20% drivetrain loss is high for a modern engine. Today manuals are more in line with 10-12% and automatics are more like 15%. The n55 I saw dyno in person put down about 268whp. My guess is crank is 315 or so.
'98 M Roadster and '13 M-Sport 328i 6mt
E36/7: Dinan/RMS stage 2+ blower@13lbs of BOOST! 18" BBS LM's, too much more to list
F30: ForgeStar F14 19's summer-OZ Superleggera 17's winter/Rogue Catback/Integral Audio/AFE intake scoop/BMS stage 1