This "model" scenario is all related to the pilot on the ground. Obviously with the pilot on the ground, the pilot will use a given power for flight into wind and will most likely notice the model fall out of translational lift on the downwind turn, so will instinctively add power to prevent this. So therefore the model pilot will be constantly modifying power to maintain level flight with reference to the ground, because that is the only reference he has.
The full size helicopter pilot flying a constant airspeed turn without reference to the ground will not require a power change just because he is going downwind, there is no (relative) downwind.
