Rocket assisted
If the aim is to simply achieve the highest speed possible in a pass through the speed trap the obvious way to do this at low cost is to put a rocket motor in the plane.
Estes rockets could be used and a for a few seconds you would have the extra thrust required.
I am assumming that the aircraft is still powered by a hot 2-stroke glow motor or electric power.
If the rocket motor is of high power a folding prop would make sense (cutting the motor after the dive) to reduce drag.
Model rockets are able to reach supersonic speeds , so these are the sort of speeds possible with this technique.
Light weight and low drag is essential althought the weight of the rocket fuel can add to the dive speed.
For more control nitrous oxide hybrid rockets can be used. These are commercially available for model use and can be throttled to some extent by varying the gas flow.
Has anyone recorded high speeds using rocket assisted model planes already ?
Please state any speeds achieved.