I am looking into getting a Fuji setup and I gotta say, the wide angle bothered me but learning about this lens and the samyang 12 it seems to be covered. I as well am not thrilled about the F4, but like you the 24Lii is my go to/favorite lens and having that view and 1.4 is pretty amazing. Still, I made due with a sigma 10-20 for a long time on my xsi, I presume that this lens is better than that.
Cover the focal lengths you love. Since I've added the 10-24mm to my gearbag I filled the gap for UWA.
I'll admit f/4 is on the slow side for my zoom demands. I just accept fuji's slow lens as "it is what it is".
I'm not too familiar with the sigma UWA on a crop. IQ wise I think the fuji UWA is very good but sunburst are "ok". I do prefer the Canon 16-35mm f/2.8mk2 as it allows me to do intentional flare. My Canon 16-35 f/4IS had a hard time to focus in the sun while I tried to do intentional flare shots compared to my f/2.8 mk2. I'm assuming the 16-35 f/2.8mk3 will have lens coatings making it difficult to do intentional flare shots.
Due to some priorities I haven't purchased an X-T2 yet. However even my 16mp fuji bodies still surprise me in the kind of IQ it produces 








