I'm just a bystander, shooting Sony, but:
The 5DIII would add 40% more resolution. The lenses you have would probably be OK. Zeiss lenses may improve sharpness somewhat but probably not much at small f-stops (f/8 - f22).
You can use Nikon's 14-24/2.8 on a canon with a Leitax mount.
Nikon D800/D800E is a nice upgrade over Canon in low ISO image quality, but I guess you can just buy a Canon 5DIII or 6D and be happy.
Check this: http://www.lensrentals.com/blog/2013/02/roger-buys-a-camera-system-finally
While I can be a "gear head" with other passions, I tend to be pretty satisfied with my photography equipment. That said, I'm feeling the urge/desire/need to invest in some new equipment.
I do photography as a passion as opposed to a profession. Ninety percent of the pictures I take would be considered landscape. I currently use a Canon 5D with the 17-40 f/4L and 70-200 f/4L lenses. My method of operation tends to be camera on a tripod, remote release, ISO at 100, either autofocus with the central point or manual focus, raw image processed in Lightroom 4/Photoshop 6 and printed 13x19 on an Epson Stylus Photo R3000.
What I'm considering is possibly a new body (Canon 5D Mark III or Canon 6D) or new lenses (most likely primes; either Canon or Zeiss in the 85mm, 35mm and 24mm range). My one caveat is that I'd like the improvement in quality to be noticeable. And by noticeable, I don't mean on charts/graphs or by other photographers. An improvement where a non photographer looking at one of my prints would be able to see the difference.