I use my 20D with a sigma 170-500 and 70-300 DO IS both with a Kenko 1.4 Pro and get autofocus with both though it can be a little hesitant with them unless there is good light/contrast. With some camera bodies like the 20D and probably the 30D this seems to work. Quality is still very good with the 70-300 but a definate drop with the Sigma. I reckon its better to leave the converter off with this lens and crop on the computer. The Canon converters will only fit with specific Canon lenses so you would have to check which ones, I don't think they would work with either of your lenses as the glass elements of the converter and lens will hit each other due to the design.
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