There was a web-chat this summer where people had the chance to ask Ralf Schumacher and Dr.Mario Theissen questions.
Well, Dr.Theissen answered one of my questions regarding the power "reserve" of the current engine by saying that they had at least 50hp that they could add without any significant changes and he was confident that we would see an impressive change once they would do a major revision!
So it's safe to assume that BMW will hold the title for strongest engine for the near future.
Now if only the stories about a V10 going into the next generation M5 will come true