Here's a sequence I did with my new 5D Mark III recently at Red Rocks Park, west of Denver. I use a hilltop radio tower and facility as a good lens testing target as it has lots of thin wires etc good for resolution testing. These were all on a tripod, but center spot auto-focus was used, no manual focus, and the auto-focusing seemed to be on the money and pretty quick, but admittedly it was a nice bright sunny day.
The first for reference is using the kit lens at 105mm, then the second is with my 70-300L @ 300mm, then the third with that lens with the Kenko 1.4X, making it 420 mm. I downsized these quite a bit for posting, but at full size, raw (especially with the new fixed DPP), these are pretty darn sharp.
I reached the total attachment file size limit with these three, but I will post a follow-up with a 100% crop of the 420 mm shot. Comparing it back to the 105 mm image (which itself is already a ~ 2X magnification vs reality) I think the reach and image quality the Kenko is providing is quite good. Now if we could just figure out how to get it to work with some of the other lenses it doesn't want to play with, I'd be totally happy with it.