I have attached a dyno sheet for the BMW E60 M5. The car was tested using 91 octane by Eurorev of Inglewood. It was tested by an independent dyno facility which is not affiliated with any European car tuner.
The gain is 32hp 24lbs tq, the only mod the car had was an ECU upgrade. The dyno result might seem high but our UK office see's gains of 42hp +, these numbers are normal for a good tuner.
If your stock M5 is reading less than 440-450hp, you might want to consider using a different dyno.
If you pair this tune up with the European Launch Control then you'll have a serious beast on your hands.