The 14L and TS-E 17 are completely different lenses. While others have already mentioned that tilt is not possible through the 14L + PP, you cannot replicate shift strictly through PP. Here is an example.
All of these images were taken with the TS-E 17 and I have not yet processed them. The first was taken unshifted. In order to get all of the building in the shot, I needed to pan up. While the Burj Khalifa does not look too bad because I centered it, you can see that the other buildings' verticals are majorly screwed up.No shift, panned up
, on Flickr
You could imagine that with a 14L, the Burj Khalifa would be quite a bit smaller on the shot as 14mm is a lot wider than 17mm.
The following shot is the same building from the same location, but this time shifted. With this shot I have leveled the camera.Shifted but not panned up
, on Flickr
The obvious difference is the verticals are now correct. Unfortunately the entire building is not within view. In order to have improved on that, I would have had to gain more height, which I was unable to do. Still, you can see that no amount of PP is going to get the second image from the first. You may be able to correct the verticals in the smaller buildings at its sides, but you will not get the same perspective in the Burj Khalifa.
Now you could argue that the two images aren't completely the same, as one of them was taken with the camera level (thus missing the top of the building) while the other was not. Here is the same view taken with the lens fully shifted and the camera panned up slightly to get the top.Shifted and panned up
, on Flickr
As you can see between the two images, the Burj Khalifa is about 30% taller on this image than on the first one. The verticals on the smaller buildings are off but not nearly as badly as in the first shot. You are not going to be able to stretch the first shot to match this one without a very noticeable quality drop. Note that a 14L would be even more severe as the Burj Khalifa would be smaller - requiring even more stretching.
I have a number of other shots that demonstrate this but from the above you can see one reason why any serious architectural photographer makes heavy use of TS lenses. The 14L is a very useful lens for extremely wide spaces (I do not own a copy, but have used one) but it has a completely different purposes from the TS-E 17 and the TS-E 24 II.