Well, you're lucky. The 17-40s I tested just never cleaned up satisfactorily until diffraction set in...
I seem to get inconsistent results with mine, if I look more closely.
Sometimes, at f/8-f/11 or even 16 I'm getting really decent corner sharpness and other times it's mush.
It seems, maybe that the times I'm getting good corner results the corners are only a few meters away, with the central subject at a similar distance. At longer distances it's very soft.
I tested the Tokina on my 5D Mark III & was not impressed. I briefly tested a Nikon 16-35 on my 5DIII & it seemed to perform respectably, but not as well as the Nikon 14-24. That being said, it may be worthwhile to revisit the 16-35 & really assess its edge-to-edge performance b/c the 77mm filter thread makes it extremely convenient for landscapes... ND filters, polarizers, grads, etc... all of which you *especially* need when shooting with the limited DR of Canon sensors.
My Nikonian friend is intent on the 16-35 VR
I just picked up a very clean used Nikon 17-35/2.8 and will begin comparing it to my 17-40/4L in some hopefully controlled situations, each lens on their native bodies (D800 and 5D2) and eeking out the max I can from each one. The loser gets listed on Kijiji.
One thing that bothers me about the 16-35 is the VR which, of course, wouldn't be engaged on a Canon body. I just feel like image stabilization adds more elements which translates to more chances of decentering/misalignment, etc. Perhaps I'm being paranoid?
I tend to agree. The more there is, the more there is to cause problems. Tamron's very decent 17-50/2.8 is an example of a good lens that went not-so-good with the addition of VC.
I love using some of my stabilized lenses for handheld walk-around shooting but when I'm on a tripod I prefer the best lens I have available for the job, usually a prime or a good performing stabilized zoom. If I'm taking the time to mount on a tripod, I'm also taking the time to get the best from my equipment in other ways.
None of the zooms I've looked at in this range are particularly good at FF border-corner performance tho.
I think Samyang's 14mm prime actually outperforms the Nikon 14-24 in the corners too, but at the expense of significant distortion in the central area which could be an issue if shooting stuff with straight lines. Water/horizons might be a problem in landscapes. 14-24/2.8 is working nicely enough otherwise, I'm really enjoying that WIDE end!