I've been following the rumors for a while. The older rumors were saying "450hp+" but in the past few months BMW marketing flacks seem to have taken every possible opportunity to temper expectations and say that it will have the similar power to the current M3 with more torque and less weight.
I don't know why it would have much less than 450hp. The new I3/4/6 engine platform was supposed to deliver up to 75hp per cylinder. They are already proving that out with the 220hp 1.5L I3 going in the BMW i8 car. They should easily be able to get 450hp out of a 3.0-3.2L I6. And why wouldn't they when the Audi RS4/5 already makes 450 and the next C63 is going to make about, if not significantly more than, 500hp.