Another demo - photograph from yesterday evening using LCC technique.
K-1,
FF mode, DA*11-18 w/o hood, f/11, focal length at the border from 15mm to 14mm - exif shows 14mm, dng raw capture
To remove vignetting in post I did a second capture using an opal pane using the same aperture. In Capture One from this capture I created a LCC profile and applied it to the original photographs, then did some tweaks. You may argue the borders at the bottom are not good enough for your taste or in total too much hassle. For some photographers - maybe. For other who like to fiddle around it brings some satisfaction and for my taste the results are easily usable.
Original capture w/o hood. Vignetting not too bad. You could easily got on with a very slight crop.
Capture of the opal pane at the original location. Vignetting and sensor spots easily recognisable.
Opal pane image after applying LCC function. The normalisation data is stored in the background.
LCC profile applied to the capture. In addition some post processing tweaks.
Compared to what astro photographers do, these steps are pretty fast done. But today most people want everything quick and easy ... and tell others that DA lenses don't work with FF sensors ... Please let people decide by themselves if they want to take the nessessary compromises into account. The DA*11-18 isn't an FF lens. That's pretty clear because to use this designation it would need to cover the FF circle for the whole zoom range. By the way - if a FF tilt shift lens shows some weaknesses when shifting into the extremes and then shows stronger vignetting I never heard somebody say that the lens isn't a FF lens.
Another example without LCC.
K-1,
(FF) Square Mode, DA15 Limited w/o hood (retracted), f/16, crop in post 4700 x 4700 px. In APS-C the square crop yields to only 3200 x 3200 px and shows significant lesser wide angle.
You can't get this image on APS-C, since the sensor size isn't sufficient. If you want a square image and crop the APS-C area accordingly, you get much lesser angle and so a different image with a lot lesser pixels! In FF mode the weakest areas are cropped away. APS-C standard mode even shows worse corners and borders on the short sides!
I got some pretty nice square images using the DA15 on the K-1 in Square Mode and a slight crop in post. If you own the lens and the K-1 I'd suggest try it yourself. You may get some nice images you otherwise wouldn't. Isn't that what we photographers are after? - Let's not only be gear heads who may again and again discuss if a lens is category APS-C or FF.
What you can use a lens for mainly depends on its image circle. The sensors aspect ratio of 3:2 by far isn't the only rectangle that you can build positioning corner points on the circles rim. In fact the APS-C sensor size restricts in a way that from the lenses' perspective isn't appropriate! So let's use FF mode if it's needed and crop in post.