This question pops out every once a while. The answer is: an electric pylon model will do 200 mph, so a jet is easily capable of much more since there is lots of power to use. We´ve measured 400 km/h with a radar in a dive for a turbine model. But, so what? Flying straight for one second (and then the model is almost too far, turn back...) is not the purpose of these models. The stress to the airframe and control surfaces increases exponentially as the speed increases. The final speed achieved is just how much the drag / power combination allows. But to keep this hobby safe we don´t really need to know how fast a turbine model is capable of flying because the answer usually is: faster than its airframe can take. Or the pilot can be in control of it.
We´re are already using 100 % runways and flying faster than a normal PPL airplane, thats enough...

BR, Tuomas Pietinen