Today we put some 6-4 model airplane props that we had lying around on it to see how they compare, and it's quite a difference. The bot was quite a bit faster, though they were spinning noticeably slower than the fan blades, indicating higher motor loads. I think with with some sort of Kort nozzle (we need prop guards anyway, why not make them aid propulsion?) these propellers will do what we need. We are planning on testing with larger fan blades and possibly with cut down airplane props.
EDIT: Here's a video of the teste with the airplane props ( the vertical thruster is still using a fan blade in the video)
http://youtube.com/watch?v=DeYNCOiSV4Q