Kenko DGX 2x Teleplus Pro 300 with 60D

Steve:  Your explanation makes perfect sense, but my Canon 450D (XSi) could autofocus quite fast on Venus with my 55-250 f/4-5.6 IS kit lens.  Even at 250 mm, the disc of Venus was tiny in the frame.  How--if at all--does that fit into the explanation?

FF