Before buying the Canon 17mm tilt-shift lens I was trying to figure out the resulting FOV that I can get with it, how does it compares to other rectilinear lenses? I have my old Sigma 12-24mm EX DG f/4.5-5.6 that used a lot on Canon 5D. Sigma is definitely worse in optic compared to the Canon 17mm TS-E, but can be used for field of view comparison.
Here are some shots I took on Sony A7R III full-frame camera mounted on tripod using Metabones V adapter.
Shot with Canon 17mm fully shifted compared to Sigma 12mm straight and tilt up shot
Canon 17mm f/4L TS-E basic shifts compared
Canon 17mm f/4L TS-E field of view compared to 12mm rectilinear lens (Sigma 12-24EX DG)
As you can see Canon provides more reach in getting tall buildings in frame, having wider FOV when shifted @10mm than regular 12mm lens. But pointing 12mm lens up and straightening perspective in post processing will provide more coverage, of course with less details and sharpness.