I'd argue that, the 17TS-E and 1.4 TC make the 24 TS-E close to redundant, and the 135 f2 and 100 Macro L are so similar I've yet to meet anybody that can reliably distinguish between images shot with either.
Many real estate shooters use both the TS-E 17 and 24mm. While the 17mm is a phenomenal lens it is not perfect. When you put on a TC the image degrades and you lose 1 stop of light so while you might get 24mm focal length you most certainly do not get a TS-E 24mm f/3.5 II.
Difference in IQ between TS-E 17mm with 1.4x-III and TS-E 24mm II is not negligible. Just take a look at sample crops...
In terms of extending the 17mm, I would think that you'd probably be better off putting it on a crop body to retain your f/4 max aperture and avoid TC-IQ-degradation but then as side effects you'd be closer to 28mm and you'd lose some high-ISO noise performance.
Well I know a lot of real estate photographers and I don't know one with the two TS-E's, heck I am unusual in my market, $500k - $2M with the 17. Those that are considering gear purchases are looking at video and this seasons darling, UAV's.
As for the IQ, well it falls into the how much is good enough, firstly I don't think the linked comparison is a good one, Bryan didn't get a good combo for that test, however, my main RE customer is a multi million dollar developer, they regularly make posters and high end printed literature out of my 17 and 1.4 TC images so the IQ is pretty stellar and it is certainly better than the MkI 24 TS-E by itself and we used those for years without complaint. Retaining f4 really isn't a factor for anything but artistic uses and I haven't done comparisons of the 17 plus 1.4 against naked on my EOS-M, maybe next time I am out with it all, I wouldn't think iso is an issue either, after all to get the best out of the TS-E's you have to use a tripod.