No. A TWT relies on the velocity of electrons inside the tube and bunches up the electrons. At least a full cycle of the signal has to be present inside the tube at once. If the velocity is about c/10 (I'm guessing this is a reasonable low end) the tube would have to be 150 meters long for a 200 kHz signal.Precision machining is required, so TWTs are expensive.
Metal detectors operate at frequencies where semiconductors can be extremely efficient -- I'm guessing better than 90% using PWM and LC filtering. There's not much utility in trying for more.