Get the Kenko 1.4x DGX (note the X at the end). Both the MC4 and PRO 300 DGX work in the same way). Cheaper, sharper, works with all lenses AND will AF on any Canon camera at any aperture if there is enough light.
Tried it with the 150-500, 50-500 and Canon 400mm and it AF on the 500D and 550D while reporting the correct aperture and length!!! Do not know why and how it does that but it works!!!
The DG ones do not AF after f5.6, ONLY the DGX do.