Interesting comparison Jonathon, and the tests on
www.slrgear.com show a similar story. Although the prime is not an L series, as it it stopped down 2 stops it is probably performing at about its optimum, where as the zoom at full open is probably not. At the other end, by f22 you are probably getting difraction setting in reducing
It would be interesting to see a comparison of both lenses at f8 which would probably show both at their best.
Also could you patch your crops together and post them so we can see thje results?