It's been nine years since I retired from 747-400 and I remember the wind readout as quite accurate on our aircraft with GPS updating of the IRU position. Indicated winds on final were a good indication as to wether one should suspect low level wind shear.
What makes this thread interesting for me is that I have built and fly a Van's RV-7A homebuilt aircraft with Grand Rapids Sport EFIS. This $3200 (USD) EFIS has wind readout that compares TAS and Magnetic Heading to GPS groundspeed and track. The remote magnetometer has errors of +3 -2 degrees around the quadrant and there is some pitot static error, both affecting the wind calculations but in general it is almost as accurate as your equipment. The Sport EFIS use's averages and is not reliable for a few seconds after a turn such as a base to final turn.
George in Langley B.C.