Image quality is a very complex matter way beyond sharpness figures. Online forums and print media are focussing on sharpness because it is easy to measure and to judge. If You go really wide angle, You'll often very likely find the sun within your frame. Here the 17-40L shines, with much better flare resistance than the 16-35L1 and even L2. Find yourself a good copy and try to attach the more efficient & elegant EW-83H lens hood (19-40mm). When bought new, You can even send your cam + lens to Canon to match them together (calibration).
As an alternative approach, one could try out prime lenses like the "king of sharpness and clarity" Olympus OM 2.8/24 MC, the old EF 2.8/24 is no slouch either and easily holds it's own compared to the 17-40L. The old primes EF 2/35 and EF 1.8/50 are much sharper then the 17-40L. Even the EF 2.8/20 USM is way better then it's webutation, and the Tokina ATX 3.5/17 is all but a hidden secret. Why prime lenses?
Prime lenses have less elements, thus less flare issues and are much more reliable when it comes to repetitive accuracy, they're simply solid partners and worth a second thought. You're much better off zooming by your own feet instead of using a 16-elements-in-11-groups-alignment-nightmare. Well, mostly.
Just my tuppence.