Rooms definitely look large when you go ultrawide, but a very wide lens might be needed at times. My last house had small bathrooms, and the 12-24 F4 Tokina on crop struggled with the small rooms. No doubt about 17 stretching out a room when used on full frame, but 20 might be useful.
I have never used a tilt and shift lens, but the cost needs to be justified, for me. If a lens will be used a lot to make money, then the expense may be justified. If not, and it's sits in the bag, then it is not money well spent.
Many zooms are not at their best at the extreme end of the range, often at the wide side. The 16-35IS seems an exception here, and might be considered. Distortion drops as you zoom to a slightly longer focal length, e.g. going to 20 on the 17-40. Something like this also happens on the 24-105L. I am a rank amateur, so take these comments with a grain of salt.