The springs on all four corners of the x-drives are interchangeable with the spring from the respective corners of the RWD. Eg: Front left= front left, Front left =/= rear left.
The reason for the difference between the rates are the motion ratios. Motion ratio measures the relative leverage a spring can have on the wheel. Since the rear spring is much further in board, it has less effect on the wheel rate. The front motion ratio is close to 1:1, whereas the rear is close to .5:1.
The spring rates are fine for a comfortable grand tourer.