Checkboard,
I think I've fallen into the trap of thinking that reverse thrust pushes against the surrounding airflow, the push being more effective at higher speed.
I thought the propulsive efficiency of the jet engine is greatest when there is a small difference in speed between the jet exhaust and the surrounding airflow, because the "push" is most effective then.
This doesn't make sense any more though!
Are you saying that thrust decreases with speed because the massflow through the engine decreases?