OK Nick, i read you. We really were talking different things....i was arguing supersonic while you were in subsonic gas flows.
However i have to slightly disagree with you again with that this type of variable area nozzle would be next to useless, because of the following reason:
(As a basis we have exhaust velocities below supersonic local to consider....on full scale as well as our turbines)
Our turbines are always a compromise between thrust and running characteristics. Best example is a turbine a friend built a while back. It produced close 9kg static without a nozzle, and ran up from idle to full in a heartbeat. After adding a really really really "tight" nozzle, it would push an amazing 13 point some kilos, however runup was veeeeery timeconsuming and involved quite a rise in temp (constant rpm gave good temps). The nozzle that will find final use gives around 11kg...with decent spoolups.
Point is: no nozzle accellerates the air "zero", thus you only have the "clean thrust" available. Addind the " fixed variable area nozzle", accellerates the air (ideally) to mach 1.0 (dynamic pressure decreasing, i know...), ruining the running capabilities.
Now adding a variable area nozzle would be a great step in the right direction, because you can have the best runup times possible, with the highest thrust possible once operational rpm is reached (nozzle squeezes tight). However large this performance increase is depends on the individual engine....but i'm guessing you could gain about 10% thrust and decrease spooltimes by about 25% simply by adding a variable area nozzle on the available turbines...
Just a rough estimate....