Appreciate your confirmation, Lnguyen1203. I could never tell for sure, from what others all over the internet have said about this combo, until your definitive statement. I'll be ordering up one Kenko Teleplus PRO 300 DGX 1.4x AF Teleconverter. Thanks!
I'm not sure if this completely true. I remember reading on TDP that the 1.4x can be mounted when the 70-300L is zoomed out a bit.
The link states:
What I just discovered, however, is that the 70-300 L's rear element retracts into the lens far enough that, at about 250mm, there is enough clearance for the 1.4x to mount.
The available focal length range is about 350-420mm. There is a physical bump at the lower end of that range with (guessing) the rubber around the edge of the extender element contacting the rear 70-300 L lens element or its edge ring. This combination indeed autofocuses on the 1D X with the new firmware.
I'll put this combination on the ISO 12233 chart testing list to see what the image quality looks like, but ... I'm not sure that I recommend mounting this combination due to potential damage the to lens. I'll try to find out what Canon says about this.
TDP also has ISO crops of the 70-300L with 1.4x, where it is shown to be softer than the 100-400L