D800 Would I gain much by going to 16-35, 17-35, 14-24 or Tokina 16-28

I'm currently shooting with a D800 and D700 and have as one of my lens a Nikkor 20-35 f2.8. I also have the 24-70 f2.8  I don't shoot a lot with an UWA, and in most cases the 20 end will do it.  But I have the urge to spend some more money, and really it's only paper if you don't spend it.

Would I be gaining anything or (very much) in image quality by going to either of the above lenses. I read that a 20 the 20-35 is probably as good as it gets, and that the 16-35 is not good in the corners at 16 or so, so what do I have to gain.

Thanks