Subdividing the plane does not matter, as Corona's displacement is adaptive and it is calculated based on the max size controlled in the Render Setup > Performance tab.
It is best to use 32-bit files for displacement - they won't give you the visible banding or stepping on the displaced geometry, if the displacement is very tall.
Obviously, the higher resolution, the more detail you will get.
Also keep in mind that displacement should usually be used for bigger details, and bump maps should be used for smaller ones.