The grey in your render is influenced by the lighting. In the renders you practically never get the color you assigned in the mat room ;)
You should use the RGB values of the color you assign to the plane (press ALT while you click on the color node) to compare with the displacement map.

But it's true, the subtract 0.5 trick doesn't work well with higher displacement values. Let's hope some future version of Poser will fix this...