Well it also depends on how the light's designed.
Most MR11 bulbs are glass, a lot of light leaks out the side and back. And they're flood bulbs.
Light and motion is, so far, the only manufacturer that designs their own custom aluminium reflectors.
I've been going on night rides with a bikeforums member and on a pitch black road, my 16 watt beam with flood reflector lights up the road enough to give about 3-4 seconds of warning for stuff at around 22-24mph. Like I said, they're just my own personal experiences, of course with glare, poorly designed reflectors (or worse yet, generic reflectors that come with most MR11 bulbs), that's bound to change.
It would be nice if people posted their experiences about how fast they can go with what type of light so we can fine tune this thing.