I was running down one of the many mountains hills around my house the other day and it became painfully clear why I don't think they're a good tool for overspeed training. While there are a handful of other reasons I'm not a huge fan of downhill running for overspeed training the biggest is the effect that it has on posture. Any time the grade of a hill is over about 3-4 degrees it makes it difficult to maintain the posterior pelvic tilt (or even neutral pelvic tilt) that is so important to proper sprinting mechanics. When pelvic tilt deviates even slightly from it's ideal (but dynamic) position it has profound affects on the athletes ability to move their limbs to the appropriate positions. You see the pelvis is the point of origin for most movements. When running downhill, the pelvis inevitably rotates anteriorly producing arch-back / butt-out postures that inevitably causes:

1. Ground contact to hasten in an attempt to improve stability,
   
2. Ground contact to occur further in front of the hips than in a normal stride pattern thus introducing big braking forces that if carried over to flat ground running could be a death sentence.
   
3. Movement of the thigh to occur further behind the trunk making the high knee positions that are so beneficial to top-end speed sprinting nearly impossible.
   

As you can see any OVER (the flat ground maximal) speed occuring as a result of the hill (it's still debatable whether you actually run faster when going down a hill...it certainly doesn't occur at all grades) comes at the cost of good sprint mechanics.  Not a good tradeoff in my opinion.