I've got a Kenko 1.4x Pro 300 DGX and a Canon MkIII 2x and use them regularly with my 5D3 and 70-200. The Kenko is excellent with hardly any drop in IQ or focussing speed. The Canon 2x is great with the 70-200 f2.8L IS II but I've not tried it on a MkI version but the Kenko is great on the MkI and MkII lenses.
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