Car and Driver weighed the F10 M5 at 4277 pounds and ran 11.9@123 in the 1/4 mile with it back in a comparison test in July of 2012. In November of 2011, Car and Driver hit 12.0@122 for the F10 M5. So, the numbers are pretty close between the M6 Gran Coupe and F10 M5 as tested by Car and Driver on different days even with the M6 coming in heavier. Keep in mind these are dual clutch transmission numbers and not those with the manual option.
It will be interesting to see if owners can match these trap speeds. IND-Distribution took their M5 to the drag strip and hit 11's but only 119 miles per hour.
As more numbers come in from magazines and owners alike we will get a better idea but it is safe to say the M5 and M6 Gran Coupe are 11 second capable cars in stock form at over 120 miles per hour (without the optional Competition Package that adds an additional 15 horsepower). The lighter F13 M6 coupe should be able to do even better than the four doors. Definitely impressive performance especially considering how heavy these cars are.