Just an aerodynamic thought from a guy who has never had reversers on his jet:
Doesn't the efflux from the deployed reversers act as a dam to block a lot of the airflow around the the engine pods and create a major change in the flow field around the aircraft? In essence, not only do you have some reverse thrust, you also have much more drag from an effectively larger, less aerodynamic aircraft. For that reason the reversers will be more effective at higher speeds. The greater the efflux from the engine, the larger the aerodynamic dam being formed around the engine and the greater the aerodynamic drag.