I'm not a huge fan of the crown risers because they:
1. Make it difficult to really punch the block pedals with a flat foot. As a result, an athlete is either forced in to a somewhat awkward position just to make full foot contact with the block pedals in the set position or must deal with the inefficiency of waiting (or not waiting) for midfoot contact to occur. In other words if the athlete does not have their midfoot on the pedals in the set position they have two options: a). push back with the hip and knee extensors and make midfoot contact right before block clearance in which case they actually have to wait for this to occur before they have a firm platform from which to apply force (because the strength of the plantarflexors would become a limiting factor) or b). push off from the toes where plantarflexor strength could again be a limiting factor. The former option might offer the benefit of a slight pre-stretch of the gastroc-soleus complex but I don't think this outweighs the negatives.
2. Cause similar problems during acceleration and MaxV. The risers position the foot in a position more likely to make contact on the forefoot but I suspect this potential advantage is outweighed by the fact that you are introducing a new potentially interfering medium through which force is applied to the ground. Also the risers create a less stable base of support to apply force. Finally, the risers add about a 1/4" to the forefoot sole of the shoe. This means an athlete can potentially make ground contact slightly sooner in the stride cycle then they otherwise would. While this might seem insignificant, at MaxV, the backward horizontal displacement of the foot with respect to the body at top speed just prior to ground contact could be significant enough to slightly change the direction of the force vector such that the athlete experiences additional braking forces.