I think if Canon knew how to make an UWA zoom lens that is sharp all the way into the corners, they would have done it already.
Nikon's 14-24 is better in the corners, but that's a benefit of the bulging front element design. The trade-off is that you can't use filters. And it's heavy.
The 16-35 is a useful event lens though, if the corners aren't important (and frequently they aren't - you don't normally put your subject in the corner of the frame).
For the moment, the TSEs or the 14mm L are your best options
