The 5D Mark III has MUCH more highlight headroom and MUCH cleaner shadows than the 5D Mark II. I've extensivly compared both and carefully studied both of these aspects for a review website.
Your camera is deffective or you are mistaken. You also might have the camera set up wrong such as auto lighting optimizer set incorrectly.
I agree... if your dynamic range is worse than the 5D Mark II, something's definitely wrong with either your camera or your processing.