From what I have read the 17-35 is a stunning lens and may well outperform the Contax Zeiss 18mm. But I am slightly cautious in taking this lens comparison at face value, the reason being that the tests were undertaken on a Canon 5D using a Nikon - Canon lens adapter. The same guy got slightly odd results when testing the Nikon 14mm that were inconsistent with the results and conclusions arrived at by other reviewers. On a subsequent test of some different lenses he changed the adapter mid test and found huge differences between the reults when using different adapters, but only on some lenses http://www.16-9.net/lens_tests/15mm_...vnikon15f.html. Hmmm.
It doesn't mean his conclusions with the 17-35 aren't valid, but I think one shouldn't read to much into them in isolation.