So far, this is sounding like a good idea......Would someone please
provide a convincing argument as to WHY I DEFINITELY should NOT try
this !

The valve will reduce the pressure to that of the hot water supply. You will
get a drizzle out of the shower head. It will be utterly pathetic and no
better than a normal gravity shower, possibly with only about 2 feet head,
if the shower is on the floor below the tank. If you don't have a pressure
balancing valve, the shower will run cold and your hot water will leave the
building via the header tank overflow.

Christian.