Does the M have larger turbos? I know stock boost for the 50i is 9.8psi and for the M is 16. Dinan lists their tune for the 50i going to 14psi which makes 496hp, whereas the M goes from 16 to 19psi with tune for 610 hp or so. Besides the boost, the other differences between the engines are pretty subtle from what I remember...larger intercooler, exhaust, etc. It's not enough to account for the 60 hp difference between a tuned 50i and a stock M, so I'm guessing it must be running bigger turbos.
More info...X6M has lower compression ratio (9.3:1 vs 10:1 for 50), makes peak power higher (6000 rpm vs 5500), and C&D lists stock boost at 17.4psi, so that could account for the difference.
2011 Nissan GTR Red/black - HP Logic tune/midpipe
2012 BMW X5 50i M-sport Dinan stage 1 Space gray/black
2011 BMW M3 convertible Jerez black/beige
Gone but not forgotten: '12 X5M, '07 AM SuperVantage, '05 Boxster S, '01 BMW M3,...
Last edited by TahoeM3; 03-08-2012 at 06:37 PM.