The 550i Xdrive only weighs 121 lbs more than the S6 (4519 lbs. vs. 4398 lbs.).
In the Car and Driver comparo, from a rolling start the M5 is almost a half second faster to 60. From 0-100 it is a full second faster and from 0-130 is is 3 seconds faster. Without launch control the S6 is definitely not in M5 territory. I like the S6 and will consider it once I see the LCI 550i Xdrive which will most definitely be no slower 0-60 than the S6 (without launch control) as the current version is within .02 of a second.