I would have to ask if you're debating between the D800 and the 5DM3, why you're spending so much money on a body. Most people don't come anywhere close to needing what these cameras offer, especially for landscapes. With landscapes why aren't you considering the D800E? You mention you want to go full frame, but why - what scenario did you have an APS-C sensor that didn't cut it? The difference in sensor size is pretty small. Are you printing larger than A3? Probably still wouldn't matter.
Gear doesn't make the shot. My evidence for this is that there are a lot of pictures taken with a D800/14-24 that don't look as good as some Mike H. shots with a Rebel/kit 18-55. A good photographer can make a good shot with any camera, but when shooting thousands of pictures maybe there are a few things that frustrate them, and they shell out extra money to remove those annoyances.
but struggle with the wide angle stuff and shoehorning enough into the image, along with the noise seems to be way too excessive at reasonably low ISO's such as 600....
A new camera won't help with "shoehorning enough into the image". That's a composition problem. Either learn to think differently about your compositions, or get a different focal length. The noise can be offset by a few different things: position yourself to where lighting is better, bracket exposures, work on your post processing techniques, try exposing differently. If you under expose your shadows, which I almost guarantee you are doing, and then try add contrast it's going to look horrible no matter which camera you use.
As for sensor dynamic range, you can check DxoMark, but be warned that the values go beyond pixel peeping into the theoretical:
http://www.dxomark.com/Cameras/Comp...S-5D-Mark-III-versus-Nikon-D800___850_795_792
I have a $500 camera that can curbstomp the D800 in resolution *and* dynamic range. I have no storm pictures to show for it (yet), because it's a pain in the ass to use! The most important thing is to get a camera that you like shooting with above everything else. If just buying landscape gear for specs and money wasn't a concern, I'd personally go:
DSLR: Nikon D800E/14-24
MILC: Sony A7R/Zeiss Distagon 21mm
I've been wondering for awhile how many chasers have ditched their dedicated camcorders and exclusively use a DSLR for both pictures and video now?
Well I don't use a DSLR for pictures, but I do keep my video separate. There were too many times I wanted pictures and video of something, and it was frustrating trying to choose which had priority.