I say, Its your car, and if you like the way the m5's look, as you should because they are sexxxxy, then you should change it. Just because a BMW is designed beautifully, doesnt mean you can't change things on it to make it your own, just think your driving a car almost exactly the same as thousands of people. It never hurts to change things up and be unique
Good luck, and I hope you are satisfied with your bumper look whichever path you take.