Those speed models with the single blade prop have a counterbalance weight hidden inside the spinner so in fact they're perfectly balanced. As they'd want to be at 35-40,000revs . They need the single blade because the engine has to rev extremely high and a 2 bladed prop would have to be too small a diameter to make the thrust needed at the same revs.

There's some photos and a video [link=http://www.flyrc.org.uk/record/control_line.htm]here[/link] of one of these single blade CL speed models setting a world record at just over 208mph....with a .15 engine. If you watch the video then turn up the sound and listen to what happens when the pipe cuts in .