Take a look at this test also http://www.dxomark.com/index.php/Publications/DxOMark-Reviews/Canon-EF-24-70mm-f-4L-IS-USM-The-ideal-standard-zoom
My experience is that it is always a good idea to refocus when you zoom a zoom lens. Focus shift is a different issue of change of focus when the lens is stopped down compared to fully open. A DSLR like the Canon will focus with the lens fully open so if you shoot at e.g. f/8 on a f/2.8 lens the focal plane may have shifted slightly on some lenses. That's focus shift.
For landscape shooting I wouldn't both about IS although nice to have. You will experience that not every shot is pixel sharp if you shoot e.g. 3-4 stops under a shutter speed that you could hand hold without IS. A tripod is needed in any case also for precise composition.
I'm not sure if their clinical approach to test lens quality has any merit.
They basically count lines and the more lines they count, the sharper is the lens.
There are things like microcontrast that is very important, and, more so, subjective/human considerations.
I have seen DxO mark rate cheap, 3rd party lenses higher than Zeiss lenses.
Photozone.de may also pixelpeepers, but they seem to include subjective impressions into their ratings.
The way the human eye sees and how to get a lens to see like that is an art in itself, and there's a reason why Zeiss can ask for these incredible prices.