Åh boy....my "moving avg." is never above 50 km/h, and that is when riding alone (not so long coffe breaks)
When riding with my friends...below 40 km/h.
I've never tried it in/on grass, but the big girl will do figure 8s in four parking spaces. The cross-over point being where the spaces all meet. The "rear brake" only feature makes it a fairly easy exercise.