Everybody's artistic taste is different. I've had the Sigma 10-20 f4-f5.6 for a coupla years now, but i don't feel any need to go to 8mm and many times, 10mm seems a bit too far. Many times i feel the 12-24 mm range suits my style of photography much better, particularly because of the 24mm end, fewer lens changes in a walk around environment.
What i've noticed with extreme FL is miniturization. Features that aren't taken close to the camera, tend to get extremely small. A second problem is that the fall-off in size for like items begins to appear freakish. Look at the motorcycle in this image
https://www.pentaxforums.com/forums/lens-clubs/107938-sigma-8-16mm-club.html#post1692324
The front tire of the motorcycle appears unusually small compared to the back tire. That doesn't make the image bad or anything, its quite a dramatic image. But there is a "look" to images taken at 8 and 10 mm that isn't always appealing. On the other hand, i have an image taken close up at 10 mm that is one of my favorites.
Its expensive to always be changing out lenses, i'd suggest you look good and hard at the type of images produced by any of these lenses, and decide what range of focal lengths you really want to push for.