Most estimate around 15% difference between wheel and crank horsepower. For the mathematically challenged out there, that would put the twin turbo V8 at somewhere around 620 horsepower.
IND's specific car has 450 miles on it, and was bone stock. They also weighed the car and found it sat at 4,313 pounds, just 25 more than BMW's official estimate. You can dissect the details of the test at M5post.com.
BMW's official horsepower rating of 560 is looking more than a little conservative right now.
(Hat tip to Kris!)
Photo Credit: iND Distribution