Interesting topic.
Found this site with MTF curves for Canon lenses:
http://www.pbase.com/samirkharusi/canon_mtf_curves
Best to select 'large' or 'original' size images to download.
Don't miss the first image which gives some idea of how to read the MTF graphs.
Also note you only need to read the results out to 13mm from the axis for APS-C sized sensors (e.g. the 400D).