What kind of mpg are you getting? As far as M6 my mpg is averaging 17. 14 in the city and around 20 on the highway. I have 2010. Its mpg seems better than what is posted for guys with 07s or 08s. Driving in p400 or 500 has not made a difference mpg stayed the same. I now only drive in p500s (once you feel the power you can't give it up). There have discussions on the M5/M6 message board about mpg in p400 or p500 and looking at responses there doesn't appear to any real difference.
2010 M6 Sapphire Black Convertible