Some MEASURED data:

Subject:
Five Jet

Thrust speed
70N 315 km/h
140N 375 km/h

Measuring device:
Real Design Flight Data Recorder

Some other interesting data:

Take off and landing speed 56 km/h
Max g-force during normal flight 6.5 g and when I tried to release other main gear when it was stucked inside the max g-force was 9.5 g
Max flying height 800 m
Max battery current when flaps were taken out 1.6 A

I think that doubling the speed is quite tough job.