New wide angle lens for FF

[Jim K]

Since you said a 24mm prime was one choice, I would go for the 24mm TS-E rather than the f/1.4 for landscapes.
I love my 24 TS-E for landscapes. The 17-40 is useful when I need to go wider for landscapes, almost always stopped down to f/8-f/11. I would have to rent a 17mm TS-E before I ever bought it. The filter problem would make me think long and hard about buying it.

[Mt Spokane Photography]

For landscape, its more of a focal length choice than a lens choice.  Typically you use them at small apertures to get depth of field, and, at f/16 for example, they are all very similar.
 
You are paying big bucks for fast lenses that you can use wide open.
 
AF speed doesn't come into play for landscape either.
 
Its possible that something else is causing your issue, such as a damaged or misaligned lens.

[scrappydog]

I would have to rent a 17mm TS-E before I ever bought it. The filter problem would make me think long and hard about buying it.

The TS-E 17 can be used with ND grads and circular filters, with limitations.  ND grads can be used by gluing a Lee 82mm wide-angle adapter to a hacked TS-E 17 cap, and then attaching a Lee filter holder to the rig.  Tilting and shifting is limited, but it works fine for straight shots.  I saw a video of a guy who did a similar hack in order to use 82mm circular filters with the TS-E 17.  It's not perfect, but it works fairly well.  There are several threads on the Fred Miranda forum discussing this hack.  If you want more info, PM me.