These were taken with my 7D on the 100-400 with a Kenko 1.4TC. You must use the outer 2 middle row focus points to get decent focusing ability. Focus is slower, you need greater light and contrast.
Question is if at this magnification, you'd see a difference with a tc and the bare lens.
To the op: I've just got a 70-300L with the Kenko (you get a working but sloooooow f8 af), so take it for what it's worth - but it's about the same iq or even a bit better than the older 100-400L. Basically, forget about using this on a crop body, you're cutting 1.6x times 1.4x away from the glass and it really shows with these midrange lenses.
I only use it to zoom in on sunsets and the like nowadays that need even more cropping, but for anything that require sharpness it's not worth switching the tc on.
I'd say the IQ is a little worse but definitely not killer loss up close. As the distance to target increases then the loss increases too.