About a month or two ago I posted a question into DT's forum about ND Graduated filters, Cokin Z-pro holder, wide angle lens 17mm at 1.6x crop sensor and vignetting / dark corners. Didn't receive an answer if there will be any dark corners or not.
To date I've had that set for a while and I thought I post an answer for my own question. So, the quick answer would be Yes, there will be dark corners with that set up!
. I've tried several holder set ups by adjusting filter holders moving them from the front to back. No good result. Then I replaced the adapter ring so that the holder would move a little more backwards. Still no good result. Even, if trying without any filters, I still get minor dark corners at 17mm @ 1.6x crop sensor (approx. 27.2mm@35mm equiv.). So, to sum up, the only way to get rid of vignetting is to use not very long shutter speed and hold the filter in hands in front of the lens. But when long exposure is wanted, a little zooming is necessary, or cloning in photoshop ;)
So, based on my experiences, even more wider filters and holders than Cokin z-pro and 100mm wide filters for 27.2mm wide lens is needed.