Yes, I have also smoothed the high poly mesh. These "normal baking problems" seem to happen to a handful of models I make, and it's always frustrating; sometimes Maya bakes normals perfectly, and sometimes craziness like this happens.
The problem I'm having seems that the Maya isn't baking the High Poly onto the low poly as a normal map, it looks like it's just baking the low poly onto a map. I've Deleted all history, smoothed all edges, reset transformations, and set the correct models to be bake, each model has a separate material. I have no idea why…