Andreas, i have to correct you on that.
On full scale airplanes, the nozzle is actuated mainly according to the EGT. The tighter you squeeze the more thrust you will get (same amount of air at higher velocity, pulse= mass x velocity²), however at higer EGT and with horrible spoolup capabilities. Variable nozzle gives optimum temperature, spoolup AND max thrust in all situations.
Same holds 100% true for model turbines. However, it is very difficult to actuate an appropriate mechanism in a jet. Then there is no ECU on the market that will allow an interface to actuate the nozzle according to current need. And last but not least: our turbines usually do not end where the real ones do: at the end of the plane. Most people utilise thrust tubes, in which the airflow has time to "adapt" to the pipe. Increasing exhaust velocity in the middle of the plane will not help much, as it will have slowed almost to the "normal" velocity by the time it exits the pipe...
A variable nozzle at the end of the pipe would help, but the losses due to the resulting high drag (=backpressure) in the pipe would probably eliminate all the advantages.

A variable nozzle would be great in terms of performance, but it would be a mechanical nightmare and an enormous effort, plus you would have to move the turbine right in the tailpipe...considering also the rather "small" speed envelope we have to cover compared to full scale aircraft, it is not "economical". It's cheaper, easier and a lot quicker to just simply put in a stronger turbine :-)

Regards
Hank