I transferred an Oatley motor (250w) to a friend's bike - which has reached speeds of 40 k/hr! - Thats the beauty of geared motors to the chainwheel :)
Edit: Has now almost reached 45 k/hr !!!
I'm very tempted to try the Crysalyte 408 (500w), ala Stoke Monkey - using it like the Oatley above. With higher gearing - it would be stupid fast! The biggest problem is creating a bracket strong enough to support it.