I think in order to get better, you would need to bite the bullet and get the 24-70L mkii and get something like a 10-22 from one of the manufacturers to provide the wider angle needs you may have. Of course, a company like sigma may release something similar in the future, but my crystal ball isn't that clear.
The sigma 18-35 is better than the canon 24-70ii on a crop body. It doesn't have much range though. its like having a 28, 35, and 50 on ff.