I believe the most powerful transponders put out 250 watts
The most powerful transponders are around 600W. And it is the peak power rather than the average which is the risk if the transponder goes faulty - not just to the aircraft but potentially also the local radar environment. It is really important that transponders can be switched off. This is all simple stuff but not really the point.
More than a decade after 9/11 I don't see much risk of "quick fixes".
More than 13 years on and the paranoia does not abate. An endless series of sticking-plaster quick fixes. When it comes to the "war on terror" there is a bizarre lack of perspective. So let's put another piece of kit on board, drill yet another hole in the aircraft skin and create yet another certification procedure (nothing on a modern airliner costs "only $150"). And without any clear idea on whether this would improve aircraft safety or by how much. Because let us be clear, what is proposed here is not a safety feature. This is a "something must be done" and "how hard can it be?" feature.