Interesting, Craig, considering the price, there's nothing to blame. The writer has stressed that if that difference worth the money.
Today, I went out and give another try on the Kenko DGX MC7 2X TC,
I mounted it on my 7D with my 300 f4. The EXIF give the correct focal length and aperture, but no mention of a converter added, (Unlike Canon EF II extender.) For small objects, it jumps a bit, and the image is not sharp. But for larger objects, it did focus with the centre point, and the image quality is fair to good. This young swallow is taken at a distance of about 15 foot, full frame, resized and slightly sharpened. But then at this distance, I think 300 f4with a 1.4X can handle it well, with a slight crop. After all, it turned out not as bad as I initially thought. If I can grab a 2X DGX pro, I will certainly try it out too, but to spend another 300+buck for an unknown is too much for me right now.