This question has been vexing me as well. My good friend (
http://en.wikipedia.org/wiki/Propeller) tells me that propellers max out around 80% efficiency. I'm afraid that we won't get anywhere near an optimum prop, so let's assume 10% efficiency with 300W of CIM, yielding 30 W. When your robot is moving at 2 m/s, 30 W should translate to 15 N, about 3 lbs. If you power your fan or fans with 2 CIMS, you get 6 lbs of thrust, which would make a nice adder to the 7.5 lbs you get from traction.
Now, if someone who knew what they were doing made the prop a mere 30% efficient, then they would have an additional 18lbs of thrust; enough to push anyone around.
Unfortunately, I have no idea where you would purchase or how you would design this propeller. There are some big RC planes out there, but I can't find much information on thrust produced or efficiency. Anybody have any leads?