is it the same principle that the full scale turkey feathers work on?  Since they have a variable exhaust opening allowing them to maintain a certain range of thrust/efflux over a wider range of speeds, also allowing the turbine to run in a relatively high loaded state....  if that makes sense. 

On the ARFs i'd think that the pipes to be designed to be a compromise between thrust/efflux velocity for a certain type of engine.   so a certain engines pipe could be improved by customizing it depending on the engine and at what range the turbine would work best at (I.e. Throttle range for optimum spool up, thrust vs efflux velocity, etc...)? 

so a to big pipe would be efficient at high throttle settings, but inefficient as you'd loose thrust in the low end/mid range?