Ok a few pictures I just tried to show the effect, actually been good to try out, easy to get lazy in your old age....
With out filter,
Now with filter
You can see as mentioned above the slight colour cast in the upper clouds.
Plus the effect of balancing out the dynamic range, I have to confess for a "mild grad" it has a bigger effect on the foreground than I expected.
These are just straight out of the camera, no level correction or bright/contrast work (I did sharpen though, as I have all options turned off in the camera, "Faithfull" setting)..
Now at 24mm with holder, grad, and no screw-in on the lens.
Note the vignetting !!
So the P system I would say is not suitable, don't think there is much point trying on the 17-40 !!!