Baz,
You could always toss the Canon
and get an Olympus 7-14mm - its near perfectly rectilinear (and so essentially without distortion) and remains so wide open.
Ahhh but its f4 (close to f3.5 by a bees whisker - remember its not a linear scale)
Also the "crop factor" is 2X due to the sensor - so thats more like 14mm focal length in 35mm terms
Or stick with the sensible advice !!
Rally