If I am understanding you correctly then what you are saying is that if you drop the grid size from 256x256 to 128x128 in the Niagara System then you would need to divide the displacements by 4 in the 128x128 case to match the displacements in the 256x256 case (ignoring the differences created by the higher frequency spectrum outside the [-128,-128] to [128,128] range).
If that is true then I think my GPU implementation is producing the expected results. I only introduced that factor to make it easier to change grid size while testing. Although, a less confusing option may have been to just multiply the displacements by GridSize / 256 for testing purposes which would eliminate my extra user parameter.