Jim, there are two possible geometries. (A) flash bracket attaches to the camera, (B) flash bracket attaches to the front of the lens.
With the right layout your claim is true, +/- perhaps half a stop, from around 1:5 to not quite 1:1 with setup (A), from around 1:2 to 2:1 for setup (B). But only for lenses with fixed focal length. Remember that some lenses, e.g., 105/2.8 and 200/5 MicroNikkor AIS, shorten focal length as they focus closer.